Synthesis and characterization of the novel rare earth orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄

2.1 Syntheses

The mixed orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ were synthesized by using stoichiometric amounts of the corresponding oxides Y₂O₃, Yb₂O₃, and Er₂O₃ (all Strem Chemicals, 99.9 %) with (NH₄)₂HPO₄ (Merck, >99.0 %) according to eq. (1) and eq. (2).

First, the starting materials were finely ground and filled into platinum crucibles. The platinum crucibles were placed into a muffle furnace (Nabertherm, Lilienthal, Germany) and heated up to 1200 °C (5 °C min^–1). After 20 h, the reaction mixture was cooled down to room temperature by switching off the heating. The mixed orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ were obtained in form of light rose and colorless crystalline materials, respectively. Both new compounds are air and water resistant.

Single crystals of the new mixed orthophosphate Y_0.5Er_0.5PO₄ were grown by the flux method [19]: 50 mol% diammonium hydrogen phosphate (NH₄)₂HPO₄, 48 mol% lead oxide PbO (Fluka, >99.0 %) and 2 mol% of the corresponding rare earth oxides RE₂O₃ (RE = Y, Er) were filled into a platinum crucible, which was covered by a second platinum crucible and placed into a tube furnace in vertical position. The heating rate was 4 °C min^–1 and the mixture was kept at 1200 °C for 30 h. Cooling was performed with a very slow rate of 0.1 °C min^–1 until a temperature of 900 °C was reached, followed by switching off the furnace. The resulting product was subsequently purified by thoroughly washing with boiling HNO₃ to remove the formed Pb₂P₂O₇ flux. The obtained crystals of Y_0.5Er_0.5PO₄ had a length of about 1 mm and were subsequently used for X-ray crystal structure analysis.

2.2 Elemental analyses

For elemental analysis of Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄, the JEOL superprobe 8100 with EDX was used leading to an average occupation of the rare earth positions of 0.51(1):0.49(1) and 0.46(1):0.54(1), respectively. From the Rietveld refinement, the corresponding ratios are 0.50(1):0.50(1) and 0.48(1):0.52(1) for Y:Er and Y:Yb, respectively. The single-crystal structure determination of Y_0.5Er_0.5PO₄ revealed an average occupation of the Wyckoff position 4a with a ratio Y:Er of 0.53(2):0.47(2). All in all, the average composition Y_0.5RE_0.5PO₄ (RE = Er, Yb) could be confirmed from all data. Thus the composition was then fixed to Y_0.5RE_0.5PO₄ (RE = Er, Yb).

2.3 Crystal structure analyses

The X-ray powder diffraction patterns of Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ were obtained by measuring in transmission geometry using a Stoe Stadi P powder diffractometer with Ge(111)-monochromatized MoK_α1 (λ = 70.93 pm) radiation. The experimental powder patterns of Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ are depicted in Figs. 1 and 2, respectively. The data of Y_0.5Er_0.5PO₄ were recorded in 2θ with steps of 0.1° from 2 to 80° and of Y_0.5Yb_0.5PO₄ with steps of 0.1° from 2 to 70° in 10 h.

Fig. 1:

Experimental (blue), calculated (red) and difference powder diffraction pattern of the phosphate Y_0.5Yb_0.5PO₄. Vertical ticks indicate the position of the reflections.

Fig. 2:

Experimental (blue), calculated (red) and difference powder diffraction pattern of the phosphate Y_0.5Er_0.5PO₄. Vertical ticks indicate the position of the reflections.

The structural refinement was performed with the program Topas 4.2 [20] by taking the crystal structure parameters of YPO₄ [21] as starting values. The crystallographic data obtained from the Rietveld refinement [22] are listed in Table 1. The new mixed rare earth orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ crystallize in the tetragonal zircon-type structure in the space group I4₁/amd (no. 141). In Figs. 1 and 2, the measured (blue) and the calculated (red, Rietveld refinement) powder patterns of Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ are displayed, respectively. The difference curves (gray) of the calculated and the measured reflections are also shown in Figs. 1 and 2 exhibiting a good agreement. The unit cell contains the phosphorus atoms on the Wyckoff site 4b, the oxygen anions in position 16h, and the corresponding rare earth atoms equally mixed in position 4a.

Additionally, a single-crystal structure determination was carried out for the compound Y_0.5Er_0.5PO₄. The data were collected at room temperature by using a Bruker D8 Quest Kappa diffractometer with MoK_α radiation (λ = 71.073 pm) and multi-scan absorption correction. Resulting data and details of the collection are listed in Table 2. According to the systematic extinctions, the space group I4₁/amd was identified. Due to the isostructural relationship between Y_0.5Er_0.5PO₄ and YPO₄ [21], the values of YPO₄ were used as starting parameters for the structural refinement (Shelxl-97 [23, 24]; full-matrix least-squares on F²). After the anisotropic refinement of all positions and a final difference Fourier analysis, no significant peaks could be identified. Positional parameters, anisotropic displacement parameters, and interatomic distances are listed in the Tables 3–7.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-430256.

3.1 Crystal structures of Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄

The crystal structures of the new mixed orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ are displayed in Fig. 3 with views in all three directions. The structure can be described by isolated [PO₄]^3– tetrahedra, which are connected by REO₈ polyhedra. In the directions a and b, a wave like ordering either of the rare earth ions or of the [PO₄]^3– units is observed. By contrast, the [PO₄]^3– tetrahedra and the rare earth ions are alternating in the c direction. In both orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄, the rare earth cations are coordinated by eight oxide anions (Fig. 4).

Fig. 3:

Crystal structures of Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ (space group: I4₁/amd (no.141)) with a view along [1̅00] (top), [01̅0] (center), and [001̅] (bottom). The PO₄ tetrahedra and the mixed occupied rare earth sites are emphasized.

Fig. 4:

The Y³⁺ and the Yb³⁺ ions in Y_0.5Yb_0.5PO₄ and the Y³⁺ and the Er³⁺ ions in Y_0.5Er_0.5PO₄ (gray/red) are coordinated by six [PO₄]^3– tetrahedra (green).

The interatomic distances obtained from the Rietveld analysis for Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ are listed in the Tables 6 and 7. They are in full agreement with previously reported bond lengths of other mixed rare earth orthophosphates crystallizing in a zircon-type structure [14]. For both compounds, the eightfold coordination of the rare earth cations by the oxide anions exhibits two specific distances. In the case of Y/Yb–O, the bond lengths are 227.2(3) (4×) and 235.9(3) pm (4×), while in the case of Y/Er–O the values are 228.2(2) (4×) and 237.4(2) pm (4×). This situation can be described as two orthogonal interpenetrating tetrahedra, which leads to a bis-bisphenoidal geometry in the form of a dodecahedron with the rare earth ion in the center.

In Table 8, the cell parameters and average bond lengths of the new orthophosphates Y_0.5Er_0.5PO₄ and Y_0.5Yb_0.5PO₄ obtained by the Rietveld powder analyses along with those of the isostructural orthophosphate YPO₄ [21] are listed. It can be seen that the cell parameters a and c as well as the cell volumes V decrease when yttrium is partially substituted by erbium/ytterbium. This is attributed to the smaller ionic radii of the eightfold coordinated rare earth ions erbium (114.4 pm) and ytterbium (112.5 pm) as compared to the ionic radius of yttrium (115.9 pm).

In Fig. 5, the lattice parameters a, c, and the cell volume V of various orthophosphates are plotted against the ionic radius of the corresponding rare earth cations. The values of the cell parameters of other orthophosphates were taken from the literature [13, 21]. The two new mixed orthophosphates fit very well in the linear regression.

Fig. 5:

Linear regression of the cell volume and the cell parameters a and c against the ionic radius of the tetragonal zircon-type RE orthophosphates.

Noteworthy, the lattice parameter a increases faster than the lattice parameter c. This can be interpreted with the arrangement of the [PO₄]^3– units and the rare earth ions in the structure as discussed.