(NH₄)₂Te₂WO₈:  A New Polar Oxide with Second-Harmonic Generating, Ferroelectric, and Pyroelectric Properties

A new polar ammonium tungsten tellurite, (NH₄)₂Te₂WO₈, has been synthesized and structurally characterized. The material was synthesized hydrothermally, using NH₄OH, WO₃, and TeO₂ as reagents, and structurally characterized by single-crystal X-ray diffraction. (NH₄)₂Te₂WO₈ crystallizes in the noncentrosymmetric (NCS) polar monoclinic space group P2₁ (No. 4) with a = 6.9716(9) Å, b = 7.0279(9) Å, c = 9.4593(13) Å, and β = 99.188(2)°. The material exhibits a two-dimensional structure consisting of WO₆ octahedra connected to TeO₄ polyhedra. Both the W⁶⁺ and the Te⁴⁺ cations are in asymmetric coordination environments attributable to second-order Jahn−Teller (SOJT) effects. (NH₄)₂Te₂WO₈ is NCS; thus second-harmonic generation (SHG) measurements were performed. SHG experiments, using 1064 nm radiation, revealed an efficiency of approximately 250 × α-SiO₂. As (NH₄)₂Te₂WO₈ is also polar, ferroelectric and pyroelectric measurements were performed. A ferroelectric hysteresis loop (polarization vs electric field) was observed, indicating the dipole moment in (NH₄)₂Te₂WO₈ is “switchable”. A spontaneous polarization (P_s) of 0.19 μC/cm² was measured. In addition, the pyroelectric coefficient (total effect), p, was determined to be −0.22 μC/m²·K at 50 °C.