Phase transitions in BaMnF4: Specific heat

Specific-heat measurements are reported for BaMn${\mathrm{F}}_4$ in the temperature region around its incommensurate phase transition. At $T_I=255.3\mathrm{}$ K a mean-field discontinuity of 0.4 cal/mol K is observed upon which is superimposed a power-law divergence of 6.4 ± 0.5 cal/mol. The shape and magnitude of the divergence are compared with the theoretical predictions of Levanyuk, Osipov, Sigov, and Sobyanin; good agreement is observed. A second, smaller $C_P$ anomaly is observed at 247.1 K of integrated magnitude 0.3 ± 0.1 cal/mol. This is compatible with observations by Levstik, Blinc, Kadaba, and Cizikov that some samples exhibit two phase transitions a few degrees apart, as evidenced by two peaks in the $b$- and $c$-axis dielectric constants. The presence of two transitions at 247 and 255 K is also confirmed by piezoelectric resonance measurements. Further evidence that the phase-transition characteristics in this material are sample dependent is provided by x-ray studies, briefly summarized.