Abstract
New phosphates, Sr1.9Cu4.1(PO4)4, Sr2Cu(PO4)2, and Sr9.1Cu1.4(PO4)7, were found in the Cu3(PO4)2–Sr3(PO4)2 system at 900°C under air. Sr1.9Cu4.1(PO4)4 crystallizes in an orthorhombic system with a=15.3695(7) Å, b=10.3652(4) Å, and c=7.9425(4) Å. Crystal structures of Sr9.1Cu1.4(PO4)7, Sr2Cu(PO4)2, and the known phase, Sr3Cu3(PO4)4, were refined by the Rietveld method from X-ray diffraction data: space group R3m, a=10.6119(1) Å, c=19.7045(2) Å, and Z=3 for Sr9.1Cu1.4(PO4)7; space group C2/m, a=11.5155(1) Å, b=5.07543(6) Å, c=6.57487(7) Å, β=106.3563(6)°, and Z=2 for Sr3Cu(PO4)2; and space group P21/c, a=9.2010(1) Å, b=4.94104(4) Å, c=17.8998(3) Å, β=122.8952(9)°, and Z=2 for Sr3Cu3(PO4)4. A second-harmonic generation study confirmed that the structures of all four compounds have the center of symmetry. Sr3Cu3(PO4)4 is isotypic with Ca3Cu3(PO4)4. Copper cations in Sr3Cu3(PO4)4 have square planar and trigonal bipyramidal coordination and strontium cations have octahedral and ninefold coordination. Sr2Cu(PO4)2 is isotypic with Ba2Cu(PO4)2. Copper cations in Sr2Cu(PO4)2 have square planar coordination and strontium cations have ninefold coordination. Sr9.1Cu1.4(PO4)7 is structurally related to β-Ca3(PO4)2 and Sr3(PO4)2. A portion of copper cations in Sr9.1Cu1.4(PO4)7 fully occupies the trigonal-distorted octahedral site M5. The structure of Sr9.1Cu1.4(PO4)7 has some disordered elements: (1) strontium cations in the M3 site statistically occupy four positions near the center of symmetry; (2) orientation disordering of the P1O4 tetrahedra is observed; (3) the M4 site is occupied by 0.05Sr2+ +0.2Cu2++0.75□; and (4) copper cations in the M4 site are slightly displaced from the threefold axis with distances between positions about 0.94 Å. Formation of solid solutions Sr9.1−xCaxCu1.4(PO4)7 (0≤x≤9.1), Sr2−xBaxCu(PO4)2 (0≤x≤2), and Sr3−xBaxCu3(PO4)4 (0≤x≤1.5) was shown.
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