--------------------------------------------------------------- --------------------------------------------------------------- A RRRR I TTTTTTTTT V V EEEEEE A A R R I T V V E A A RRRR I T V V EEE AAAAAAA R R I T V V E A A R R I T V EEEEEE 2K --------------------------------------------------------------- --------------------------------------------------------------- USER'S GUIDE Le Mans, first version: 1984 Last update: January 2000, all previous 4 programs gathered in one, greatly simplifying the use. A. Le Bail Universite du Maine Laboratoire des Fluorures ESA CNRS 6010 Avenue O. Messiaen 72085 Le Mans Cedex 9 FRANCE E-mail : either armel@fluo.univ-lemans.fr or alb@cristal.org or cristal@cybercable.tm.fr or lebail@univ-lemans.fr ----------------------------------------------------------------- ----------------------------------------------------------------- SHORT INTRODUCTION: ARITVE is a program for modelling amorphous structures by a Rietveld-type refinement of the atomic coordinates and cell parameters taken from a starting crystalline model. The data fitted are the S(2t) interference functions (see § A). Up to three S functions can be simulated simultaneously, either neutron or X-ray data or both together with different wavelengths. The imaginary part of the X-ray diffusion factors is correctly treated (I hope). A non-zero S(0) cannot be reproduced. From the crystalline model, a powder diffraction pattern is generated by the summation of all contributing reflections (hkl) with a Gaussian profile shape. The FWHM (Full Width at Half Maximum) angular dependence may follow a microstrain-type description (variation as tan(theta)), but the instrumental resolution and eventually a size effect are simply taken into account by a Cagliotti-type expression. Not every model will give a "good" fit by using this method. Only "good" model(s) will give "good" fit(s). An eventually "good" model may be soon detected : low Rp (Reliability on profile, according to the Rietveld original definition) is obtained BEFORE refinement. It is recommended not to try models inconsistent with already known structural aspects (distances, coordinations, density...). ARITVE can only improve (by refinement) a yet "good" model. Test files are provided that correspond to the fit of glassy SiO2 neutron and X-ray data published in : "Modelling the silica glass structure by the Rietveld method," A. Le Bail, J. Non-Cryst. Solids, in press (1995). However, the fit does not correspond to the final result in the P212121 space group but to an intermediate result with higher symmetry (P213). Note that ARITVE is not a convivial program easy to manipulate. Do not expect to get a result before some weeks on your own problem although you may install and apply the program on the test data in a few minutes. Some french text may have been forgotten somewhere. This package has not been extensively tested by many users, bugs are possible. ----------------------------------------------------------------- ----------------------------------------------------------------- CONTENT A- How to run the program: definitions of files and parameters B- Strategy B-1- The model B-2- An order to respect for parameter refinement C- What can be expected from ARITVE D- How ARITVE could be eventually improved E- Final recommendations F- BIBLIOGRAPHY - REFERENCES G- LIST OF CAUSES OF MAJOR PROBLEMS ----------------------------------------------------------------- ----------------------------------------------------------------- A- How to run the program: definitions of files and parameters Explanations are given corresponding to the example of the test files: ARCAR.DAT data file with commands for structure refinement ASIO2N.NOR neutron interference function of SiO2 ASIO2X.NOR X-ray interference function of SiO2 ARITVE.ZIP contains also : ARITVE.EXE : the axecutable for Windows 95/NT/98 ARITVE.F : the Fortran source code LICENSE.HTML : the GNU General Public License ARITVE.TXT : this manual I the general case, one .dat file with instructions will be needed as well as up to 3 .nor files containing interference functions. The data included in the SiO2 .NOR files are from: J.H. KONNERT & J. KARLE, Acta Cryst. A29, 702 (1973) for X-rays and P.A.V. JOHNSON, A.C. WRIGHT & R.N. SINCLAIR, J. Non-Cryst. Solids, 58, 109 (1983). for neutrons The starting model is that of the Carnegieite structure: F.W. BARTH & E. POSNJAK, Z. fur Kristallogr. 8, 376-385 (1932). But the starting coordinates and cell parameters are from the (may be false) description of a high-cristoballite form with the carnegieite model: T.F.W BARTH, American Journal of Science, Serie 5, 23, 350-356 (1932). (see also Wyckoff, Crystal Structures, Volume I). WARNING : These "interference functions" are the so called S(2t) i.e. the equivalent of S(Q) with a contant 2*theta step (the diffracted intensity, properly corrected, divided by <f**2>). Here, the test files are given at an arbitrary scale (multiplied by an arbitrary constant). ASIO2N.NOR is given there, the first line must contain three values : nbmes 2t Step unformatted nbmes = number of intensity data 2t = starting two theta angle Step = constant step in two theta degrees Then the nbmes intensities are given (unformatted) 230 0.00 0.40 0.00000E+00 0.65620E+03 0.68291E+03 0.73245E+03 0.80835E+03 0.93140E+03 0.11210E+04 0.13651E+04 0.16390E+04 0.19135E+04 ...... 0.57189E+04 0.57198E+04 0.57212E+04 0.57232E+04 0.57258E+04 0.57290E+04 0.57327E+04 0.57370E+04 0.57418E+04 0.57474E+04 0.57535E+04 0.57602E+04 0.57674E+04 0.57753E+04 The same for ASIO2X.NOR : 230 0. 0.4 0. 93. 232. 421. 639. 833. 1042. 1255. 1461. 1674. 1890. 2106. 2321. 2558. 2800. 3054. 3368. 3760. 4287. 4959. 5787. 6738. 7703. 8556. 9142. 9386. 9251. 8808. 8203. 7549. ...... 5195. 5195. 5202. 5215. 5230. 5254. 5296. 5350. 5401. 5469. 5538. 5620. 5694. 5793. 5890. 5984. 6059. 6133. 6206. 6268. 6335. 6398. 6457. 6500. 6542. 6566. 6578. 6589. 6596. 6583. the ARITVE program is started by ARITVE entry file (no extension)?? ARCAR The .DAT file must be prepared as follows: The test file ARCAR.DAT: line ! CONTENT 1. ! IF AMORPHOUS SiO2 WAS TYPE CARNEGIEITE P213 2. ! 90 2 13 1 0 0 0 0 0 0 3. ! SiO2-N 3. ! SiO2-X 4. ! 1 4 5. ! 4 2 6. ! 0.5000 0.7093 7. ! 0.5 0.8 0.8 0.05 8. ! P 21 3 9.1 ! 0.4149 0.5803 9.1 ! 0.0704 0.0060 9.2 ! 6.2915 2.4386 3.0353 32.3337 1.9891 0.6785 1.5410 81.6937 1.1407 0.0817 9.2 ! 3.0485 13.2771 2.2868 5.7011 1.5463 0.3239 0.867 32.9089 0.2508 0.0106 10.1 ! SI1 10.2 ! 1 0.25500 0.25500 0.25550 0.04000 10.1 ! SI2 10.2 ! 1 -0.00800 -0.00800 -0.00800 0.04000 10.1 ! O1 10.2 ! 2 0.12500 0.12500 0.12500 0.04000 10.1 ! O2 10.2 ! 2 0.66000 0.66000 0.06200 0.12000 11. ! 60.97914 313235.4 5000.0 106958.6 11. ! 48.11535 566242.9 10000.0 279894.0 12. ! 7.16000 7.16000 7.16000 90.00000 90.00000 90.00000 13.1 ! 2 13.2 ! 0 440 13.2 ! 9200 17500 13.1 ! 2 13.2 ! 0 600 13.2 ! 9200 17500 14. ! 1.0 14. ! 1.0 15. ! asio2n.nor 15. ! asio2x.nor 16. ! 81 81 81 0 16. ! 91 91 91 0 16. ! 101 101 101 0 16. ! 111 121 131 0 17. ! 11 17. ! 21 18. ! 31 0 41 18. ! 51 0 61 19. ! 71 71 71 0 0 0 ----------------------------------------------- | ------------------------------------------- | | | PARAMETER DEFINITIONS | | | ------------------------------------------- | ------------------------------------------------ LINE 1 : TEXT --> TITLE FORMAT 20A4 --------- LINE 2 : NCYCLE,NPAT,MAXS,LG,IF1 to IF6 FREE FORMAT --------- NCYCLE --> NUMBER OF REFINEMENT CYCLES NPAT --> NUMBER OF PATTERNS (MAXIMUM 3) MAXS --> NUMBER OF REFINED PARAMETERS LG --> CODE TO SEE EVENTUALLY Iobs AND Icalc ON THE SCREEN: IF 0 --> YOU SEE THEM IF 1 --> YOU DO NOT INTEREST : ESTIMATE THE SCALE FACTOR WHEN THE REFINEMENT IS STARTING AND ALSO THE FWHM PARAMETERS IF1 to IF6 --> Codes for optional output files IFn=0 no output IFn=1 output IF1 : *.out file, contains contributing hkl to each point IF2 : bidon.hkl, the list of hkl and multiplicity IF3 : *.pre, intermediate file before refinement IF4 : bidon.imp, intermediate file IF5 : bidon2.imp, intermediate file IF6 : *.lt4 contains observed and calculated data LINE 3 : PTEXT --> ONE TITLE FOR EACH PATTERN FORMAT 4A4 --------- =====> NPAT LINES (max NPAT = 3) LINE 4 : KXR(1) KXR(2) KXR(NPAT) FREE FORMAT --------- =====> NPAT VALUES ON ONE LINE KXR --> CODE FOR THE PATTERN TYPE: 1 : NEUTRONS 4 : X-RAYS LINE 5 : NA,KL FREE FORMAT --------- STRUCTURE INDICATORS : NA --> THE NUMBER OF INDEPENDENT ATOMS CONTAINED IN THE ASYMMETRIC UNIT (Max = 50) KL --> THE NUMBER OF ATOMS HAVING DIFFERENT DIFFUSION FACTOR, EITHER NEUTRONS OR X-RAYS (Max = 4) LINE 6 : DLABDA(1) .... DLABDA(NPAT) FREE FORMAT --------- WAVELENGTHS IN ANGSTROMS FOR EACH PATTERN NPAT VALUES ON ONE LINE LINE 7 : RELAXC,RELAXB,RELAXS,RELAXH --------- RELAXATION FACTORS APPLIED BY MULTIPLICATION ON THE SHIFTS AFTER EACH REFINEMENT CYCLE RELAXC --> CONCERNS ATOMIC COORDINATES RELAXB --> UNUSED RELAXS --> CONCERNS SCALE FACTORS AND OCCUPANCY FACTORS RELAXH --> CONCERNS CELL PARAMETERS AND FWHM LINE 8. : Space group ---------- Spacegroups are defined as in Lazy-Pulverix : KLAUS YVON, WOLFGANG JEITSCHKO AND ERWIN PARTHE J.APPL.CRYST. (1977), 10, P 73-74 S P A C E G R O U P SYMBOLS DO N O T INCLUDE THE STAR PRECEEDING SOME OF THE SYMBOLS. THE STAR INDICATES CENTROSYMMETRIC SPACE GROUPS WHICH HAVE BEEN DESCRIBED WITH SEVERAL SETTINGS. THE PROGRAM GENERATES ONLY THE SETTING WITH THE CENTRE OF SYMMETRY AT THE ORIGIN OF THE UNIT CELL. W A R N I N G A SYMBOL THAT DOES NOT FIGURE IN THIS LIST MAY YIELD WRONG EQUIPOINTS. TRICLINIC P 1 P -1 MONOCLINIC P 2 P 21 C 2 P M P C C M C C P 2/M P 21/M C 2/M P 2/C P 21/C P 21/N C 2/C THE POINT POSITIONS GENERATED FROM THESE SYMBOLS CORRESPOND TO THE MONOCLINIC SETTING WITH B AS UNIQUE AXIS (ALPHA=GAMMA=90.) ORTHORHOMBIC P 2 2 2 P 2 2 21 P 21 21 2 P 21 21 21 C 2 2 21 C 2 2 2 F 2 2 2 I 2 2 2 I 21 21 21 P M M 2 P M C 21 P C C 2 P M A 2 P C A 21 P N C 2 P M N 21 P B A 2 P N A 21 P N N 2 C M M 2 C M C 21 C C C 2 A M M 2 A B M 2 A M A 2 A B A 2 F M M 2 F D D 2 I M M 2 I B A 2 I M A 2 P M M M *P N N N P C C M *P B A N P M M A P N N A P M N A P C C A P B A M P C C N P B C M P N N M *P M M N P B C N P B C A P N M A C M C M C M C A C M M M C C C M C M M A *C C C A F M M M *F D D D I M M M I B A M I B C A I M M A TETRAGONAL P 4 P 41 P 42 P 43 I 4 I 41 P -4 I -4 P 4/M P 42/M *P 4/N *P 42/N I 4/M *I 41/A P 4 2 2 P 4 21 2 P 41 2 2 P 41 21 2 P 42 2 2 P 42 21 2 P 43 2 2 P 43 21 2 I 4 2 2 I 41 2 2 P 4 M M P 4 B M P 42 C M P 42 N M P 4 C C P 4 N C P 42 M C P 42 B C I 4 M M I 4 C M I 41 M D I 41 C D P -4 2 M P -4 2 C P -4 21 M P -4 21 C I -4 M 2 P -4 C 2 P -4 B 2 P -4 N 2 P -4 M 2 I -4 C 2 P -4 2 M I -4 2 D P 4/M M M P 4/M C C *P 4/N B M *P 4/N N C P 4/M B M P 4/M N C *P 4/N M M *P 4/N C C P 42/M M C P 42/M C M *P 42/N B C *P 42/N N M P 42/M B C P 42/M N M *P 42/N M C *P 42/N C M I 4/M M M I 4/M C M *I 41/A M D *I 41/A C D TRIGONAL P 3 P 31 P 32 R 3 P -3 R -3 P 3 1 2 P 3 2 1 P 31 1 2 P 31 2 1 P 32 1 2 P 32 2 1 R 3 2 P 3 M 1 P 3 1 M P 3 C 1 P 3 1 C R 3 M R 3 C P -3 1 M P -3 1 C P -3 M 1 P -3 C 1 R -3 M R -3 C ALL R-SPACE GROUPS REFER TO THE HEXAGONAL SETTING HEXAGONAL P 6 P 61 P 65 P 62 P 64 P 63 P -6 P 6/M P 63/M P 6 2 2 P 61 2 2 P 65 2 2 P 62 2 2 P 64 2 2 P 63 2 2 P 6 M M P 6 C C P 63 C M P 63 M C P -6 M 2 P -6 C 2 P -6 2 M P -6 2 C P 6/M M M P 6/M C C P 63/M C M P 63/M M C CUBIC P 2 3 F 2 3 I 2 3 P 21 3 I 21 3 P M 3 *P N 3 F M 3 *F D 3 I M 3 P A 3 I A 3 P 4 3 2 P 42 3 2 F 4 3 2 F 41 3 2 I 4 3 2 P 43 3 2 P 41 3 2 I 41 3 2 P -4 3 M F -4 3 M I -4 3 M P -4 3 N F -4 3 C I -4 3 D P M 3 M *P N 3 N P M 3 N *P N 3 M F M 3 M F M 3 C *F D 3 M *F D 3 C I M 3 M I A 3 D LINE 9.1 : KL VALUES (SEE LINE 5) OF ---------- - FERMI LENGTHS - NEUTRON CASE OR OF - DELTA F" - X-RAY CASE (IMAGINARY DISPERSION CORRECTION) FOR EACH PATTERN (NPAT LINES) FREE FORMAT THE ORDER OF THE KL VALUES MUST BE CONSISTENT WITH THAT GIVEN LATER WHEN THE PATTERN IS A X-RAY PATTERN, A 9.2 LINE MUST FOLLOW LINE 9.2 : A1 B1 A2 B2 A3 B3 A4 B4 C DELTAF' FREE FORMAT ----------- 9 COEFFICIENTS FOR ANALYTICAL APPROXIMATION TO THE X-RAY SCATTERING FACTORS FOLLOWED BY THE REAL PART OF THE DISPERSION CORRECTION -------------------------------------------- LINES 10.1 AND 10.2 :: ATOMIC PARAMETERS -------------------------------------------- TO BE GIVEN NA-TIME (SEE LINE 5) LINE 10.1 : IDENTIFICATION ASCII FOR THE N-IEME ATOM ------------ FORMAT A4 LINE 10.2 : NTYP,X,Y,Z,NOCCUP ------------ FREE FORMAT NTYP --> ORDER NUMBER OF THE CORRESPONDING DIFFUSION FACTOR X,Y,Z --> REDUCED ATOMIC COORDINATES NOCCUP --> SITE OCCUPANCY ALL VALUES CAN BE MULTIPLIED BY A CONSTANT, THIS CAN BE TUNED ALSO BY THE SCALE FACTOR. LINE 11. : SCALE U V W FREE FORMAT ----------- ===> FOUR VALUES FOR EACH PATTERN SCALE --> SCALE FACTOR U --> LINE-WIDTH FACTOR V --> LINE-WIDTH FACTOR W --> LINE-WIDTH FACTOR FWHM == SQRT(U*Tg**2(theta) + V*Tg(theta) + W) and the FWHM is the full width at half maximum in (2-theta degrees)*100 LINE 12. : A,B,C,ALPHA,BETA,GAMMA FREE FORMAT ----------- DIRECT CELL PARAMETERS npat groups of lines 13. LINE 13.1 : Nex = number of excluded zones Free Format ----------- for the nth pattern LINE 13.2 : ilow, ihigh Free Format ----------- = the low and high limits of the excluded zone in degrees 2*theta*100 Nex lines 13.2 have to be given If Nex=0 for the nth pattern, give no 13.2 line Then, npat groups of lines 14. have to be given LINE 14. : Iscale Free Format ----------- Iscale = a multiplicative factor used to eventually modify the intensities of the nth interference function. npat groups of line 15. LINE 15. : name.nor 20A1 ----------- = name of the nth datafile LINES 16 TO 19 : CODES TELLING THE PROGRAM TO REFINE OR NOT THE PARAMETERS -------------------------------------------------------------------------- APPEARING IN THE SAME ORDER AS THE PARAMETERS LINES 10.2, 11 AND 12 THE KEY OF THESE CODES IS GIVEN HERE: CODE = [ M * 10 + ABS(SM) ] * SIGN(SM) WHERE : M --> M-IEME REFINED PARAMETER SM --> MULTIPLYING FACTOR APPLIED TO THE SHIFT FOUND FOR THE M-IEME PARAMETER AT EACH REFINEMENT CYCLE EXAMPLES : IF CODE = 101 , this is the 10th PARAMETER, THE SHIFT WILL BE MULTIPLIED BY +1 IF CODE = 100.5 , this is the 10th PARAMETER, THE SHIFT WILL BE MULTIPLIED BY +0.5 IF CODE = 0 THIS PARAMETER IS NOT REFINED IT IS POSSIBLE TO ASSOCIATE SOME PARAMETERS IN A SIMPLE WAY: FOR INSTANCE X, Y=1/2+X, Z THE CODES FOR X AND Y SHOULD BE THE SAME BECAUSE THE SHIFTS ON X AND Y=1/2+X WILL BE THE SAME : 101 101 111 FOR INSTANCE X, Y=-X, Z THE CODES FOR X AND Y COULD BE 101 AND -101 OTHER EXAMPLE: X, Y=2X, Z THE CODES FOR X AND Y COULD BE 100.5 AND 101. OR 101. AND 102. BECAUSE THE SHIFT ON Y IS TWICE THAT ON X THE ORDER FOR THE NUMBER OF THE PARAMETERS IS UNIMPORTANT. iT IS JUST RECOMMENDED THAT THE MAXIMUM NUMBER IS EQUAL TO MAXS (LINE 2) FORMAT ALWAYS FREE FOR THE CODES LINE 16. : FOUR CODES POUR X,Y,Z,NOCCUP FREE FORMAT ---------- NA LINES TO BE GIVEN (put always the NOCCUP code = 0) LINE 17. : ONE CODE FOR SCALE FACTOR FREE FORMAT ---------- =====> NPAT LINES LINE 18. : THREE CODES FOR U, V, W FREE FORMAT ---------- =====> NPAT LINES LINE 19 : 6 CODES FOR THE CELL PARAMETERS FREE FORMAT ---------- ============>>>>>>>> ATTENTION THESE CODES APPLY TO A RECIPROCAL METRIC TENSOR THE CONSTRAINTS WHICH SHOULD BE APPLIED ARE EXAMPLIFIED HERE: EXAMPLE OF CODES FOR CUBIC : 11 11 11 0 0 0 TETRAG : 11 11 21 0 0 0 TRIGONAL OR HEXAGON : 11 11 21 0 0 11 <---------------- ORTHOR : 11 21 31 0 0 0 MONOCLI : 11 21 31 0 41 0 TRICLIN : 11 21 31 41 51 61 MONOCLINIC IS FOR beta DIFFERENT FROM 90 DEGREES THE RHOMBOHEDRAL CASE IS ALWAYS TO BE TREATED IN TRIGONAL SETTING _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ OUTPUT FILES CREATED BY ARITVE: ARCAR.IMP : results cycle after cycle ARCAR.PAR : intermediate and final new parameters, can be included in ARCAR.DAT in place of the old ones ARCAR1.PRF : ASCII file containing observed and calculated S(2t) for the first pattern can be seen by DMPLOT ARCAR2.PRF : ,, for the second pattern OPTION OUTPUT ARCAR.OUT : Intermediate file giving hkl attribution to each point ARCAR.LT4 : file containing analogous information as the .PRF files BIDON.DAT : Intermediate file BIDON.IMP : Intermediate file BIDON2.IMP : Intermediate file BIDON.HKL : hkl and multiplicity --------------------------------------------------------------- --------------------------------------------------------------- CPU TIME NEEDED: For the test file calculation with ARITVE (90 refinement cycles) on a Pentium II 266MHz, the total CPU time was 2 minutes and 14 seconds. --------------------------------------------------------------- --------------------------------------------------------------- ARCAR.PAR output file obtained after 1 cycle SI1 1 0.25522 0.25522 0.25572 0.04000 SI2 1 -0.00776 -0.00776 -0.00776 0.04000 O1 2 0.12533 0.12533 0.12533 0.04000 O2 2 0.66016 0.65969 0.06207 0.12000 61.67587 351787.0 5000.0 106291.8 48.46439 593292.5 10000.0 280832.2 7.15890 7.15890 7.15890 90.00000 90.00000 90.00000 That file will give the new parameters after each cycle. In principle, you have to select the last result and insert it in the *.dat file in place of the old parameters. --------------------------------------------------------------- --------------------------------------------------------------- Part of the ARCAR1.PRF file obtained after 1 cycle This file is suitable for use as data for a graphical output by the DMPLOT program (.prf format) Adaptation is easy since the .PRF file contains: 2theta-end 2theta-initial Step number of phase , number of points two unused values All observed S(2t) and then all calculated ones: 91.600 0.000 0.400 1 230 1 1 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 2968. 3686. 4804. 6347. 7884. 8705. 8489. 7535. 6301. 5231. 4480. 3943. 3539. 3233. 3008. 2867. 2809. 2879. 3443. 4701. ...... 5920. 5946. 5978. 6014. 6056. 6101. 6148. 6195. 6242. 6286. 6325. 6358. 6381. 6394. 6394. 6380. 6355. 6321. 6279. 6232. 6181. 6128. 6072. 6016. 5960. 5904. 5849. 5797. 5747. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 2496. 3717. 5105. 6471. 7578. 8211. 8257. 7744. 6827. 5732. 4676. 3808. 3196. 2842. 2712. 2767. 2976. 3313. 3752. 4257. ....... 6444. 6438. 6419. 6386. 6343. 6292. 6235. 6174. 6114. 6056. 6001. 5953. 5910. 5875. 5845. 5821. 5802. 5785. 5771. 5757. 5742. 5726. 5709. 5690. 5671. 5651. 5633. 5618. 5608. 0. --------------------------------------------------------------- --------------------------------------------------------------- B- Strategy This part explains how to proceed. A very important point is that your interference functions must be accurate and go to as large Q as possible, up to the end of modulations. But the program works with S(2t), t for theta (constant wavelength data), see A.C. Wright, J. Non-Cryst. Solids 123 (1990) 129. If you have only one data set (i.e. one neutron pattern, or one X-ray), then it would be better if your material was a mono-atomic one. The ideal case for a di-atomic material is that you have three neutron interference functions with isotopic substitutions corresponding to large scattering length variations (nevertheless, why not adding the X-ray interference function). Of course, the more you have data to fit, the more your model will be credible. THE "PERFECT" FIT OF ONE INTERFERENCE FUNCTION IN THE CASE OF A POLYATOMIC AMORPHOUS MATERIAL IS NOT SUFFICIENT FOR CLAIMING THAT YOUR MODEL IS A POSSIBLE ONE ("PERFECT" meaning Rp<1% may be). B-1- The model As the Rietveld method, applied to structure refinement from powder diffraction data, ARITVE needs a starting structural model. The choice of one or several models belongs to the user's responsibility. The source of possible models is, as should be evident, to be found in some well known structural data base (ICSD, Cambridge, Pearson's Handbook of Crystallographic Data for Intermetallic phases...). It is also an evidence that the models have to be chosen among isoformula crystallized compounds with atoms behaving similarly in crystal chemistry. However, all amorphous materials have not a formula as simple as "SiO2". Thus sometimes several compositions can be chosen in the glass forming domain in order to reproduce exactly the formula of crystallized compounds known to coexist in the diagram with the amorphous ones. That is to say, for structure modelling of amorphous materials, THE STRATEGY MAY BEGIN AT THE SYNTHESIS STAGE. Suppose that possible crystalline models exist but with different kind of atoms (ex: BeF2 instead of SiO2), then you should modify the cell parameters of the model in a proportional way to BE COHERENT WITH THE ATOMIC DENSITY OF YOUR AMORPHOUS COMPOUND. B-2- An order to respect for parameter refinement: Once the model has been chosen and all the files prepared, you cannot start by refining all parameters. Do not expect that any case will work as well as the test case where all parameters are refined together and Rp decreases regularly from 8.14 and 5.35 to 3.39 and 2.90 respectively for neutron and X-ray data after 90 cycles. If you try that directly, the calculation may "explode". Remember that the test case is a "good" one... First you must choose a set of U V W parameter (in order to broaden sufficiently the reflection profiles) and perform some cycles refining the scale factors only till stabilization of the Reliability factor(s) Rp. If there is not any similitude at this stage between the observed and calculated interference function, you have not A large chance to succeed. You must have Rp<20 or even 10% at this stage. Second: adjust U V and W manually or try to refine U and W with V fixed to a low value (corresponding to the resolution function of the apparatus). The alternate refinement of U, W and then the cell parameters can be attempted (or simultaneous, but this is dangerous because they are highly correlated). Third, the atomic coordinates together with the scale factor should be refined. If some atom has a very low scattering length for all the interference functions simultaneously, then you should not refine its coordinates. One must note that the thermal motion effects are considered as included in the disorder effect simulated by the line-width variation. The above suggestions may not work because the whole process is very unstable... At all stages it is possible to compromise the next step by too much a deviation of one parameter or another. With the exception of the scale factors, the relaxation factors have to limit heavily the variation of the refined parameters. Easy fall down to false minima cannot be excluded. Remember that each model will have a Rp limit. Process slowly in introducing new refined parameters. Do not keep new parameters that led to divergency. However, sometimes Rp may increase before to decrease. When significant changes on the cell parameters have been observed, the whole first step (preparation of .hkl and .pre files) must be done again to be consistent with the new values. --------------------------------------------------------------- --------------------------------------------------------------- C- What can be expected from ARITVE Rietveld application to amorphous materials should be considered as one particular approach to their structure modelization: essentially a way to test quickly an idea about the possibility that there could be some similitude between the mean organization in a given amorphous material and the strict tridimensional one of a given crystalline structure. This method can be used to eliminate wrong models. The method is however insufficient to establish the validity or unicity of a model, even if the fit is quite "good". In all published applications (see the list of references at the beginning of this guide), "good" fit (say Rp < 3%) could be obtained with models that showing a few unrealistic distances between some atom-pairs. However, the mean distances for particular types of pairs were generally credibles. After all, this holds also for other methods of simulating amorphous material structures, such as molecular dynamics or model-building followed by relaxation or Reverse Monte Carlo. With this program I tried a lot of models for SiO2 (X-ray + neutron data simultaneously fitted) without obtaining a fully satisfying one... The best of the more simple ones is proposed as a test of the program, derived from the carnegieite structure. --------------------------------------------------------------- --------------------------------------------------------------- D- How ARITVE could be eventually improved For those who are not satisfied by the modest performances of ARITVE, some works are suggested: The program efficiency would be improved by: - Using constraints on distances during refinement, thus ensuring the respect of external indications (from EXAFS measurements or from well established crystal chemistry). - Generation of the new hkl list at each cycle following the eventual cell parameters variation. - Etc... --------------------------------------------------------------- --------------------------------------------------------------- E- Final recommendations - You must not limit yourself to the use of this program, even if the results seem satisfying. Try other methods and programs as the RMC of McGreevy, molecular dynamics. - Any consequence of using this program and consequence of result interpretation should not be considered as depending of the program author responsibility. - This program has not been tested extensively, problems may occur... ----------------------------------------------------------------- ----------------------------------------------------------------- F- BIBLIOGRAPHY - REFERENCES References for the original Rietveld method are: H.M. RIETVELD ACTA CRYST. 22,151-152 (1967) H.M. RIETVELD J. APPLIED CRYST. 2, 65-71 (1969) ARITVE was built from the multipattern Rietveld program of: M.W THOMAS & P.J. BENDALL ACTA CRYST. A34, 5351 (1978) The original text (in French) describing the principle of the ARITVE program and some applications may be found in: A. LE BAIL, THESE DE DOCTORAT D'ETAT, LE MANS (1985) The method together with an application was first introduced at the Thirteenth International Congress of Crystallography, 9-18 Aout 1984, Hambourg: A. LE BAIL & C. JACOBONI, ACTA CRYST. A40, Suppl. C477 (1984) Applications and some discussions on the method can be found in: A. LE BAIL, C.JACOBONI & R. DE PAPE, J.DE PHYSIQUE, COLL. C8, 46, 163-168 (1985) A. LE BAIL, C. JACOBONI & R. DE PAPE, MATER. SCI. FORUM 6, 441-448 (1985) M. LEBLANC, G. FEREY, J.M. GRENECHE, A. LE BAIL, F. VARRET, R. DE PAPE & J. PANNETIER, J.DE PHYSIQUE, COLL. C8, 46, 175-179 (1985) A. LE BAIL, B. BOULARD & C. JACOBONI, MATER. SCI. FORUM, 19-20, 127-136 (1987) M. MARET, P. CHIEUX, J.M. DUBOIS & A. PASTUREL, J. PHYS.: CONDENS. MATTER 3, 2801-2817 (1991) The glassy SiO2 modelling is in : "Modelling the silica glass structure by the Rietveld method," A. Le Bail, J. Non-Cryst. Solids, in press (1995). The parallelism with modelling crystallite size/microstrain in Rietveld analysis is described in: A. LE BAIL, NIST SPECIAL PUBLICATION 846, 142-153 (1992) The method has been cited in some review articles: A. C. WRIGHT, J. NON-CRYST. SOLIDS 106, 1-16 (1988) A. C. WRIGHT, J. NON-CRYST. SOLIDS 123, 129-148 (1990) and may be others... In these articles, the method was classified in the same group as the reverse Monte Carlo method(s), may be improperly. In the first version, a strict tan(theta) dependence constraint was applied to the FWHM in order to follow exclusively a microstrain effect. However, for d(hkl) > dmin, the FWHM was forced to be equal to that calculated for dmin (with dmin near of 1.9 Angstroem). Otherwise the line profiles became Dirac peaks when tan(theta)--> 0. This was a way to take account of the instrumental resolution function and eventually a size effect. The present version gives similar results with a Cagliotti-type FWHM variation law expected to take account of all possible effects. This law is the classical one used in Rietveld-type refinement. A demonstration that this law is able to model microstrain and size effect together with the instrumental resolution may be found in: R.A. YOUNG & P. DESAI, Arkiwum Nauki o Materialach, 10, 71-90 (1989). ----------------------------------------------------------------- ----------------------------------------------------------------- In case of use, references that may be cited are : A. LE BAIL, ARITVE User Guide, Universite du Maine, France (2000). or "Modelling the silica glass structure by the Rietveld method," A. Le Bail, J. Non-Cryst. Solids, in press (1995). --------------------------------------------------------------- --------------------------------------------------------------- G- LIST OF CAUSES OF MAJOR PROBLEMS: -------------------------------- -Your data go at too large angle. It would be better if you limit them to nearly 90 2-theta. You could work on your whole dataset by a regeneration at a shorter wavelength using interpolation. The problem may be that to simulate your data at large angle, the addition of the contributing reflexions occuring up to 180 degrees 2-theta are unsufficient. -U (and may be V or W) is too large and the last reflections are contributing yet to many of the last points of your pattern(s). -Non-respect of maximal limits of the program: 3 patterns 60000 (hkl) per pattern 20000 overlapping (hkl) at a diffracting angle 1200 points per pattern 70 refined parameters 4 different atom-type in your sample ....... -Lack of experience in refining crystalline structures. It is not recommended to study amorphous without a good knowledge of crystalline compounds and of techniques to determine and refine structures from single crystal or powder diffraction data. -You have not read this guide... --------------------------------------------------------------- --------------------------------------------------------------- GOOD LUCK