Low-temperature dielectric properties and phase transition in BaMnF4

The temperature dependences of the static dielectric properties of BaMn${\mathrm{F}}_4$ were investigated from 4 to 300 K at 1 kHz to 9.8 GHz with emphasis on the behavior near the structural phase transition at 247 K. Anomalies in the dielectric constants $\epsilon _{}^{\prime }{}_i{}^{}$ and losses $tan{\delta }_i$ at the transition were observed along the major crystallographic directions, the main feature being a $\lambda$-shaped anomaly in ${\epsilon }^{\prime }$ along the polar $a$ axis. This anomaly persists to very high frequencies (9.8 GHz). Anomalies were also observed in $\epsilon _{}^{\prime }{}_a{}^{}$ at ∼ 30 K and in $\epsilon _{}^{\prime }{}_b{}^{}$ at ∼ 70 K. These are believed to be related to the onset of spin ordering at these two temperatures. The effects of pressure on the dielectric properties and transition temperature $T_c$ were also investigated. $T_c$ increases nonlinearly with pressure at low pressures, reaches a maximum and then decreases at higher pressures. This is discussed briefly in terms of the lattice-dynamical origin of the transition. The crystal is highly anisotropic and this is reflected in the $\epsilon _{}^{\prime }{}_i{}^{}$. This anisotrophy is due to the lattice contribution to $\epsilon _{}^{\prime }{}_i{}^{}$ and specifically the relevant lattice resonance frequencies. The over-all results and their implications are discussed.