[Wien] Query regarding elastic constants of SnSe2
shamik chakrabarti
shamik15041981 at gmail.com
Mon Aug 12 15:20:50 CEST 2024
Dear Prof. Fecher,
Thank you for your elaborate reply Sir. It will be
very helpful.
with regards,
On Mon, 12 Aug 2024 at 14:11, Fecher, Gerhard <fecher at uni-mainz.de> wrote:
> 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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--
Dr. Shamik Chakrabarti
Research Fellow
Department of Physics
Indian Institute of Technology Patna
Bihta-801103
Patna
Bihar, India
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