Low Temperature Specific Heat of Sodium-Vanadium Bronzes (NA(X)V(2)O(5)), CADMIUM(1-X)MANGANESE(X)SELENIDE, MERCURY(1-X)MANGANESE(X)SELENIDE, and of Potassium.
The specific heat of Na(,x)V(,2)O(,5), x = 0.2, 0.25, 0.3, 0.36, and 0.41, and of pure V(,2)O(,5) single crystals was measured between 0.5 and 50 K in zero magnetic field as well as in fields of different intensities up to 2.8 Tesla. The specifice heat of pure V(,2)O(,5) shows a well defined maximum in C/T('3); this indicates the presence of low-lying modes which can be described by Einstein oscillators. The lowest Einstein frequency (nu)(,E) obtained from the specific heat data is 76 cm('-1) while the Debye temperature (theta)(,D) = 164 K. Adding sodium to V(,2)O(,5) enhances the specific heat below 7 K considerably. The excess specific heat is linear in temperature and is magnetic field dependent and can be described by a simple model of many three level systems with spin S = 1. However this model is at the moment not understood theoretically. This is the same difficulty encountered in spin glasses which also contain an excess specific heat rather linear in temperature over a considerable temperature range. For the insulating Cd(,1-x)Mn(,x)Se single crystal with composition x between 0.005 and 0.5, the specific heat was measured between 0.5 and 50 K, and the low field susceptibility between 4 and 50 K. The samples with x (GREATERTHEQ) 0.2 show a spin-glass transition at low temperature. The phase diagram for the boundary of the paramagnetic and spin-glass region is presented. The excess specific heat in zero magnetic field of all the paramagnetic samples show a pronounced maximum at (TURN) 0.3 K and a broad shoulder at (TURN) 0.9 K. This sharp maximum is the consequence of single Mn ions for which crystal field has removed part of the six-fold degeneracy of the lowest ('6)S(,5/2) state. To produce a maximum in the specific heat at 0.3 K, the energy level splitting needed is about fifty times larger than the splitting found by EPR measurements for samples with much lower Mn concentration. The broad shoulder is a consequence of ions which are paired and have an Heisenberg exchange energy J = -1.75 K. The specific heat of the two mercury compounds Hg(,1-x)Mn(,x)Te and Hg(,1-x)Mn(,x)Se is almost identical. The phase diagram for Hg(,1-x)Mn(,x)Se is more complete and is different from that of Hg(,1-x)Mn(,x)Te. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.
- Pub Date:
- March 1982
- Physics: Condensed Matter