Questions tagged [lattice-dynamics]

Questions related to the lattice dynamics of solids.

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7
votes
0answers
109 views

Why volume of unit cell is not the same as cubing the lattice constant?

I have a cubic-FCC with a=b=c and $\alpha$=$\beta$=$\gamma$=$90^o$. As far as I understand the volume of this cubic unit cell should be a cube of the lattice constant. But this not the case I am ...
8
votes
2answers
376 views

Pressure applied DFT calculations in Quantum ESPRESSO

I am trying to perform the pressure-induced DFT calculations in Quantum ESPRESSO. For this, I have calculated the lattice constant of the cubic structure by vc-relax...
9
votes
1answer
124 views

Is there an upper limit of temperature after which the quasi-harmonic approximation (QHA) fails?

I can't seem to articulate what made me ask this question but it sprang up when I asked: What does it mean to say a material is 'anharmonic'?. I've heard before that the electronic entropy ...
14
votes
2answers
950 views

What does it mean to say a material is 'anharmonic'?

I understand that a harmonic picture of the potential in a material isn't enough to study lattice dynamics thoroughly. The quasi-harmonic approximation is a good workaround and helps incorporate ...
9
votes
0answers
157 views

Phonon density of states as a function of frequency vs energy

Lattice dynamics codes like Phonopy output density of states, $g_{(\omega)}$, as a function of frequency. Since $\int g_{(\omega)}d\omega$ is the number of states ...
12
votes
1answer
81 views

Phonon density of states in statistical thermodynamics

Phonon density of states (DoS), $g_{(\omega_i)}$, help understand the distribution of states across frequencies of vibration. $\int g_{(\omega_i)}d\omega_i =$ the number of states between $\omega_i$ ...
16
votes
2answers
549 views

How to deduce phase transitions from a phonon calculation?

I came across the concept of using phonons to establish a material's dynamical stability, based on whether or not imaginary frequencies are present in its phonon band structure. What I am struggling ...
12
votes
2answers
112 views

Can we do anything to reduce the number of displacements in a frozen phonon lattice dynamics study?

I realize that the more ordered the supercell is, the lower the number of displacements. Perhaps, we could say the same with respect to symmetry. Is there anything that can be done to reduce the ...
19
votes
1answer
237 views

What do phonon dispersion (or lattice dynamics) studies include?

I use the codes phonopy and phonopy-qha for phonon dispersion studies. phonopy-qha gives ...