Vibronic-itinerant electronic transition

The FM phase is also stabilized by pressure. In an x = 0.15 sample, we [199] found a transition from vibronic to itinerant electronic behavior in the pressure range 5 < P < 6 kbar and the temperature range Tco < T < Tc it marks the FV-FM transition. A neutron-diffraction study [220] of an x = 0.165 sample showed that pressure stabilizes the R-rhombohedral phase relative to the O phase. We have also found that Tc jumps discontinuously by about 15-20 °C not only at the first-order O —O transition, but also at the FV-FM and the 0 -R transitions [199, 220a]. In addition, the slope dTc/dP drops discontinuously on crossing each transition. 

Okuda et al. [221] have shown that the Dehye temperature increases with x in the range 0.12 < x < 0.30, saturating at a normal value for the perovskite structure above x = 0.30, which indicates that strong electron-lattice interactions persist in the FM phase out to x = 0.30. The electronic specific heat is only slightly enhanced as x decreases to x = 0.17, but it drops sharply to zero with decreasing x in the interval 0.15 < x < 0.17 where there is a transition from itinerant to vibronic electronic behavior. 

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