[en] A single crystal of a new sodium calcium iron (III) phosphate, Na₄CaFe₄(PO₄)₆, has been synthesized by a flux method and characterized by X-ray diffraction, Moessbauer spectroscopy and magnetic susceptibility measurements. The compound crystallizes in the monoclinic space group C2/c(a=12.099(5) A, b=12.480(5) A, c=6.404(2) A, β=113.77(3) deg. , Z=2, R₁=0.022, R_w2=0.066). The crystal structure belongs to the alluaudite type, characterized by the X(2)X(1)M(1)M(2)₂(PO₄)₃ general formula. The open framework results from Fe₂O₁₀ units of edge-sharing FeO₆ octahedra, which alternate with M(1)O₆ octahedra (M(1)=((1)/(2))Na+((1)/(2))Ca) that form infinite chains. These chains are linked together through the common corners of PO₄ tetrahedra yielding two distinct tunnels of sodium cation occupation. This compound is antiferromagnetic with a Neel temperature of 35 K. Moessbauer parameters are consistent with the structural results

[en] Phases related to the different forms of K²SO⁴ have been studied in the Na³PO⁴-Sr³(PO⁴)²-EuPO⁴ system by DTA and X-ray diffraction. They appear within the range of composition limited by the solid solutions NaSr_1-xEu_2x3/vacant_x4/PO⁴ and Na_2+ySr_2(1-y)Eu_y(PO⁴)². At high temperature the α-K²SO⁴ structure type seems to be adopted, while at low temperature the various phases crystallize with structures related to the ß-K²SO⁴ type. (author)

[en] The use of europium as a local structural probe allows the various phases appearing in the NaCaPO₄-Na₃Eu(PO₄)₂ and NaSrPO₄-Na₃Eu(PO₄)₂ systems to be detected. The broadening of the europium emission lines in going from the calcium to the strontium phases illustrates the ease of displacement of the PO₄ groups. (Auth.)