| Summary: | In this study, a new series of phosphors, Ca<sub>9−<i>x</i></sub>Zn<i><sub>x</sub></i>Gd<sub>0.9</sub>(PO<sub>4</sub>)<sub>7</sub>:0.1Eu<sup>3+</sup> (<i>x</i> = 0.00–1.00, step d<i>x</i> 0.05), was synthesized, consisting of centro- and non-centrosymmetric phases with <i>β-</i>Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>-type structure. Crystal structures with space groups <i>R</i>3<i>c</i> (0.00 ≤ <i>x</i> < 0.35) and <i>R</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula><i>c</i> (<i>x</i> > 0.8) were determined using X-ray powder diffraction and the method of optical second harmonic generation. In the region 0.35 ≤ <i>x</i> ≤ 0.75, phases <i>R</i>3<i>c</i> and <i>R</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula><i>c</i> were present simultaneously. Refinement of the Ca<sub>8</sub>ZnGd(PO<sub>4</sub>)<sub>7</sub> crystal structure with the Rietveld method showed that 71% of Gd<sup>3+</sup> ions are in <i>M</i>3 sites and 29% are in <i>M</i>1 sites. A luminescent spectroscopy study of Ca<sub>9−<i>x</i></sub>Zn<i><sub>x</sub></i>Gd<sub>0.9</sub>(PO<sub>4</sub>)<sub>7</sub>:0.1Eu<sup>3+</sup> indicated the energy transfer from the crystalline host to the Gd<sup>3+</sup> and Eu<sup>3+</sup> luminescent centers. The maximum Eu<sup>3+</sup> luminescence intensity corresponds to the composition with <i>x</i> = 1.
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