[Wien] Query regarding elastic constants of SnSe2
Fecher, Gerhard
fecher at uni-mainz.de
Mon Aug 12 10:41:14 CEST 2024
Dear Morteza
It is interesting that your paper J All Comp 735 (2018) 569-579 gives the correct answer for trigonal compounds namely 6 cij for the present case,
but IRElast does not make use of it !
Your answer
"Hexagonal compounds have five independent elastic constants (C66=(c11-c12)/2)"
does not fit to the question, as the compound belongs to a trigonal (NOT hexagonal) crystal system, so what ?
(Also note that the distortions for trigonal and tetragonal systems may be different, even though the number of cij is the same.)
Your script should not look for the H lattice but for the space group, which it finds correctly from the struct file as I tested !
The default number of distortions needs to be much larger than 5 otherwise the polynomial fits will have a large uncertainty
(see remarks on ElaStic below and my previous answers) this can be tested easily when calculating standard deviations and errors .
Dear Shamik,
here is a more detailed answer, please read and understand it
Your queries were;
(1) I have obtained c55 (with 5 constants) while literature shows c44 & c66
(with 6 constants). Why?
(2) There is a big mismatch in the value of C33. Why?
Answer (1)
Both is wrong, SnSe2 should have 6 (not only 5) independent coefficients and c66=(c11-c12)/2 is not independent.
But that's not all directly your fault, see above, and you should have asked why c14 is not calculated, however c55=c44 !
Answer (2)
see answer (1), and nobody knows the details about what you have done.
You may tell me that you used a different structure, but then the comparison to literature is nonsense.
You mentioned the following values:
c11=94.575, c12=27.192, c13=23.2555, c33=102.7 and c55=22.384
from literature without reference
c11=111, c12=33.39, c13=33.08, c33=22.63, c44=44.48, c66=38.80
these values are obviously taken from Y. Javed et al., Int J. Thermophysics 43 (2021)
and were calculated with VASP (Monkhorst-Pack, 7x7x5, Perdew-Zunger LDA)
I guess the authors have known that they need 6 different cij
and used only the 6 largest one and this results in nonsense (see below).
For crystal systems see https://en.wikipedia.org/wiki/Space_group
check also for difference between crystal family and crystal system
hexagonal systems have a 6-fold, and trigonal have a 3-fold symmetry.
Trigonal crystal systems may belong to rhombohedral (143-148) or hexagonal (149-167) lattice systems.
see https://en.wikipedia.org/wiki/Crystal_system#Crystal_classes
>From J.F. Nye Physical Properties Of Crystals, Their Representation by Tensors and Matrices, Oxford (1985)
one finds from Table 9 for the number of independent cij or sij:
isotropic 2
cubic 3
hexagonal 5
trigonal 6 or 7, depending on space group
tetragonal 6 or 7, depending on space group
orthorhombic 9
monoclinic 13 depending on orientation
triclinic 21
see also R. Golesorkhtabar et al, Comp Phys Comm 184 (2013), in particular Tables 1 and 4.
Here you will find also a detailed description of the deformation types and magnitudes.
This paper will answer also the follow up question about the order of the fit (e.g. Figs. 3 and 4).
Note that they used 51 distortions and 6th order polynomial fits !
>From the Materials Project webpage one finds the following elastic tensors
https://legacy.materialsproject.org/materials/mp-665/
Space group: P -3m1 [164]
Point Group: -3m
Crystal System: trigonal
see also Pearsons Crystal Database for the structure of SnSe2.
Gamma, 10x10x6, PBE
Stiffness Tensor Cij (GPa)
85 21 3 0 0 0
21 85 3 -0 0 0
3 3 5 0 0 0
0 -0 0 2 0 0
0 0 0 0 2 0
0 0 0 0 0 32
Compliance Tensor Sij
12.8 -3 -5.9 -4.1 0 0
-3 12.8 -5.9 4.1 0 0
-5.9 -5.9 229.1 0 0 0
-4.1 4.1 0 611.4 0 0
0 0 0 0 611.4 -8.1
0 0 0 0 -8.1 31.5
from the non vanshing s14 it is clear that also c14 is not vanishing !!
AS IRElast does not calculate for trigonal systems one finds that
VASP delivers in detail (note the different k-mesh)
Monkhorst-Pack, 8x8x4, PBE
from OUTCAR
NOTE THIS IS in kBar NOT in GPa, and some coefficients have different indices !!
TOTAL ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX 1009.9710 280.4181 153.8533 0.1441 -4.8747 -0.0733
YY 280.4181 1012.6904 153.8225 2.7466 4.8397 -0.5768
ZZ 153.8533 153.8225 281.1224 0.0784 0.0167 -0.7403
XY 0.1441 2.7466 0.0784 366.7170 0.2335 -4.9295
YZ -4.8747 4.8397 0.0167 0.2335 95.3172 -0.4235
ZX -0.0733 -0.5768 -0.7403 -4.9295 -0.4235 91.8435
--------------------------------------------------------------------------------
from vaspkit
Read Elastic Constants from the ELASTIC_TENSOR.in file if it exists (GPa).
100.9971 28.0418 15.3853 -0.4875 -0.0073 0.0144
28.0418 101.2690 15.3822 0.4840 -0.0577 0.2747
15.3853 15.3822 28.1122 0.0017 -0.0740 0.0078
-0.4875 0.4840 0.0017 9.5317 -0.0423 0.0234
-0.0073 -0.0577 -0.0740 -0.0423 9.1844 -0.4929
0.0144 0.2747 0.0078 0.0234 -0.4929 36.6717
constants for the trigonal crystal system with space group 149 to 167, here R -3m (164)
c11 c12 c13 c14 0 0.00000
.. c22=c11 c23=c13 c24=-c14 0.00000 0.00000
.. .. c33 0 0.00000 0.00000
.. .. .. c44 0.00000 0.00000
.. .. .. .. c55=c44 c56=c14
.. .. .. .. .. c66=(C11-c12)/2
using symmetry the tensor reduces to
Elastic Constants (GPa).
101.13305 28.04180 15.38375 -0.48813 0.00000 0.00000
.. 101.13305 15.38375 0.48813 0.00000 0.00000
.. .. 28.11220 0.00000 0.00000 0.00000
.. .. .. 9.35805 0.00000 0.00000
.. .. .. .. 9.35805 -0.48813
.. .. .. .. .. 36.54562
Ciao
Gerhard
DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."
====================================
Dr. Gerhard H. Fecher
Institut of Physics
Johannes Gutenberg - University
55099 Mainz
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von morteza jamal via Wien [wien at zeus.theochem.tuwien.ac.at]
Gesendet: Sonntag, 11. August 2024 07:30
An: Shamik Chakrabarti; wien at zeus.theochem.tuwien.ac.at
Cc: morteza jamal
Betreff: Re: [Wien] Query regarding elastic constants of SnSe2
Hi
Please check parameters ( lattice parameters, XC, kpoint mesh , ...) which you have used for calculations with them in SnSe2 paper.
As i could find
You:
a= 3.89218 Ang, c=6.582611
XC= PBE
IF your system is Spin-plorized your command run must be: runsp_lapw ....
paper:
a=3.811 Ang, c=6.137
XC=LDA
Kpoint mesh= 15×15×11
DId you checked forces on atoms after minimization??
Hexagonal compounds have five independent elastic constants (C66=(c11-c12)/2)
And
Tetragonal compounds have 6 or 7 independent elastic constants.
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