Single Crystal Tungstates for Resonance and Emission Studies

Abstract

A number of single crystal tungstates have been prepared for paramagnetic resonance and emission studies. Compositions such as monoclinic MgWO₄, ZnWO₄, CdWO₄, and tetragonal CaWO₄, SrWO₄, and BaWO₄ containing paramagnetic ions of the transition and rare earth series have been prepared. They were grown by dissolving the constituent oxides into an equimolar mixture of Na₂WO₄ and WO₃ at 1100 to 1250°C and cooling at 2 to 3°C per hour to 700°C. These crystals offer the advantages of one environment for the paramagnetic ions present, freedom from strain (hardness equal to that of iron), absence of random ordering effects, minimal effects from nuclear magnetic moments, and chemical stability. Large zero field splitting are observed in the monoclinic crystals, making them of interest for studies at high microwave frequencies. The tetragonal compositions are promising at the lower microwave frequencies and have shown very narrow resonance lines. Line widthṡ as low as 1.5 Mc have been obtained in CaWO₄:Mn. Na_0.5Y_0.5WO₄ crystals, which have the scheelite structure, were also grown. Random ordering effects of monovalent sodium and trivalent yttrium, however, result in a nonhomogeneous electrostatic environment throughout the crystals and hence cause broad paramagnetic resonance lines.