SPAM: Re: [sdpd] structure of Zn2GeO4 reg

[Marco Sommariva <marco.sommariva...@mater.unimib.it>]

Dear Anoop,
I'm also a PhD student, and I try to tell you what I know: in
rhombohedral system you have the primitive unit cell (a rhombohedral
cell) which has a=b=c, and angles all equal, not necessarily equal to 90
deg (a generic rhombohedre); but you can also consider the centered
hexagonal cell, with 2 centering points in fractional coordinates (2/3,
1/3, 1/3) and (1/3, 2/3, 2/3); this cell has a=b, and c arbitrary, with
alpha=beta=90, gamma=120. In general you can always consider the
primitive cell (in this case the rhombohedral cell, R cell), with a
smaller volume, or the centered cell (in this case the hexagonal cell,
which has a triple volume of the primitive rhombohedral cell). 
In general, often, only the centred cell shows the full symmetry of the
lattice, that's probably why in the JCPDS card you found you saw data
only of the centered hexagonal cell.

To go from the hexagonal cell to the R cell, you have to apply the
transformation matrix T = [2/3 -1/3 -1/3
                           1/3 1/3 -2/3
                           1/3 1/3 1/3]
whose determinant is 1/3 (the ratio of the volume cells.

Maybe some others more experts than me can be clearer, 
Good job, marco!




 

Il gio, 2006-02-23 alle 12:13, anoop menon ha scritto:
> hello all
>   i am a research student from Cochin University of Science and  ttechnology, Kerala, India. I have some basic doubts regarding the  structure of zinc germenate.
>   in the JCPDS card it is given as rhombohedral with space group R-3 but the a,b,c values are given as a=b=14.231 and c=9.53
>   but normally rhombohedrals have a=b=c and alpha =beta=gamma not equal to 90.
>   why this discrepancy, is it due to any specific reason. please help me  in thei gardssince i am not an expert in crystallographic analysis.
>   thankyou  all in advance
>   anoop menon
>   
> 
>   Anoop Menon 
> Research Scholar, 
> Optoelectronics devices lab, 
> Dept. of Physics, 
> CUSAT, 
> Kochi-22
>    
>   PH: Mob-9447033331
>   visit us on http://physics.cusat.ac.in/oed/lab_oed.htm
>   
> 
> 
> 
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-- 
Marco Sommariva
Department of Materials Science
University of Milano - Bicocca
Via R. Cozzi 53 - 20125 MILAN, ITALY
Phone +39.02.6448.5141 (5142)
Fax   +39.02.6448.5400
e-mail: marco.sommariva...@mater.unimib.it



 
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