Low Temperature Phase Transition of an Orthorhombic Tungsten Bronze Ferroelectric Sr_2-xCa_xNaNb₅O₁₅

The dielectric and pyroelectric properties of a new ferroelectric crystal with orthorhombic tungsten bronze structure Sr_2-xCa_xNaNb₅O₁₅ have been investigated in the temperature range of 300 to 10 K. Based on anomalies in the properties and by use of Curie's principle of symmetry, a ferroelectric-ferroelectric phase transition between mm2 and m is found to occur at about 75K. On cooling through the transition, the spontaneous polarization deviates by about two degrees from the two-fold axis 001 toh0l direction in the mirror plane.