Effect of pressure on the chemical isomer shift

2 The second-order Doppler shift calculated for a Debye model with 9d = 200 K and an atomic mass of 55-9. 

The chemical isomer shift is the sum of two terms, the one being proportional to the 5-electron density at the nucleus, and the other being the second-order Doppler shift. Both of these will be influenced by a change in pressure, but it is the overall effect on the former which is the more significant. 

The measured pressure coefficient at constant temperature can be represented [11] by 

The first term is the effect of pressure on the electron density at the nucleus and the second term describes the relativistic effect of altering the mean vibrational energy. The latter can be simply expressed, using the Debye model for the vibrational energy of vjb = 3A r[l + - (Qu/T)- — as 

The quantity —(9 In 0d/9 In F)r can be calculated from Gruneisen s constant y and (3 In V/dP) is the compressibility. The magnitude of the expression will be small for a low Debye temperature of the lattice, and is generally less than the electronic term [11, 12]. 

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