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Two new glaserite-type orthovanadates: Rb2KDy(VO4)2 and Cs1.52K1.48Gd(VO4)2

The title compounds have the glaserite structure type. The DyO6 or GdO6 octahedra share their three six vertices with six VO4 tetrahedra, three of which are upward and the other three down. The remaining cations are localized between the sheets resulting from the tetrahedra-octahedra linkage via common vertices.

From a crystallographic point of view, the related (A,A 0 ) 3 Ln(XO 4 ) 2 compounds with A,A 0 = K, Rb and Cs adopt three structure types. The first is a monoclinic system, space group P2 1 /m, represented by the phosphate K 3 Nd(PO 4 ) 2 . The second one is trigonal, space group P3, represented by K 3 Lu(PO 4 ) 2 , while the third one is also trigonal but in space group P3m1 and represented by the glaserite K 3 Na(SO 4 ) 2 . The present work is a continuation of our structural investigations by X-ray diffraction of the (A,A 0 ) 3 Ln(XO 4 ) 2 system where A,A 0 = K, Rb and Cs, Ln = rare earth and X = P, V, As (Rghioui et al., 1999(Rghioui et al., , 2002(Rghioui et al., , 2007. The present paper reports the synthesis and the crystal structure determination of the title compounds by X-ray diffraction at room temperature and vibrational spectroscopy.

Figure 1
Layer of VO 4 tetrahedra linked to DyO 6 octahedra by vertex sharing in the structure of Rb 2 KDy(VO 4 ) 2 .

Figure 3
View along the c axis of a layer in the structure of the title compounds, showing the cavities in which the K/Rb + (or K/Cs + ) cations are located.

Figure 4
Three-dimensional view of the crystal structure showing Rb + (or Cs + ) cations between the layers stacked along the c axis.
sharing an apex with DyO 6 or GdO 6 octahedra in such a way as to form a layer parallel to the ab plane, as shown in Fig. 1. Three of the six VO 4 tetrahedra surrounding each DyO 6 or GdO 6 octahedron are oriented upwards and the other three down. The concatenation of these polyhedra delimits large tunnels and cavities of site symmetry 3m and 3m in which are located rubidium and a statistical mixture of rubidium and potassium atoms (Fig. 2).
The coordination polyhedron of the mixed site is formed by ten oxygen atoms belonging to three edges, one face and one vertex of five VO 4 tetrahedra as shown in Fig. 3. The K/Rb-O distances range from 2.681 (8) to 3.312 (7) Å . The twelve oxygen atoms surrounding the rubidium atom form an irregular cuboctahedron with Rb-O distances varying between 3.133 (2) and 3.4649 (3) Å . The main interatomic distances and angles are compatible with the values quoted in the literature (Gagné & Hawthorne, 2016;Gagné, 2018).
The three-dimensional structure consists of a basic tetrahedral-octahedral framework, forming layers that stack along the c-axis direction, as shown in Fig. 4. In glaserite-like structures, the large cations are located between the layers in channels running along the a-and b-axis directions and the average size cations are located in the cavities (see Fig. 5).

Vibrational spectroscopy
The Raman and infrared spectra for Rb 2 KDy(VO 4 ) 2 are shown in Figs. 6 and 7, respectively. Their band assignments given in Table 1 are based on previous works for homologous vanadates (Rghioui et al., 1999(Rghioui et al., , 2012Benarafa et al., 2009). The stretching modes of (VO 4 ) 3À anions are usually found in the region 950-700 cm À1 . The peaks observed in the Raman spectrum at 935, 875 and 740 cm À1 as well as the corresponding bands in the infrared spectrum at 925, 830 and 755 cm À1 are all attributed to the symmetric (VO 4 ) 3À and the asymmetric (VO 4 ) 3À vibration. The bending vibrations of (VO 4 ) 3À are seen in the range 390-310 cm À1 . As in previous works (Rghioui et al., 2012), the separation between the symmetric and asymmetric bending can not be identified in the vibrational spectra. The bands lying between 230 and 95 cm À1 in the spectra are assigned to the lattice vibrations. They are due to the VO 4 rotation and to the VO 4 , K + , Rb + and Dy 3+ translation modes. A comparison of the Raman and infrared bands shows that they are not coincident. This fact confirms the centrosymmetric structure of Rb 2 KDy(VO 4 ) 2 vanadate.

Synthesis and crystallization
Single crystals of Rb 2 KDy(VO 4 ) 2 and Cs 1.52 K 1.48 Gd(VO 4 ) 2 were synthesized by the flux method using a mixture of K 2 CO 3 , Rb 2 CO 3 (or Cs 2 CO 3 for the Gd compound), Dy 2 O 3 (or Gd 2 O 3 ) and V 2 O 5 corresponding to 1 mol of K 2 RbDy(VO 4 ) 2 (or Cs 1.52 K 1.48 Gd(VO 4 ) 2 and 1 mol of Three-dimensional perspective view along c axis of the crystal structure of Rb 2 KDy(VO 4 ) 2 .
Rb 3 VO 4 (or Cs 3 VO 4 ). The reagents were ground in an agate mortar and placed in a platinum crucible. The temperature was raised slowly to 873 K and maintained for 24 h, permitting the carbonates to decompose. A second treatment at the melting temperature of 1273 K was performed, followed by slow cooling at a rate of 4 K h À1 to 673 K and then quickly to ambient temperature. Each thermal treatment was interspersed with grinding. The obtained product was then washed with distilled water in order to eliminate the flux. The resulting product contained single crystals of a suitable size for the X-ray diffraction study.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. In the refinement procedure, the substitutional occupation of the mixed sites was freely refined and restricted to the occupancy of one site for Cs 1.52 K 1.48 Gd(VO 4 ) 2 but restricted to 0.5:0.5 for Rb 2 KDy(VO 4 ) 2 . ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006) for (I); ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008) for (II). For both structures, software used to prepare material for publication: publCIF (Westrip, 2010).