Mixed-metal phosphates K1.64Na0.36TiFe(PO4)3 and K0.97Na1.03Ti1.26Fe0.74(PO4)3 with a langbeinite framework

K1.65Na0.35TiFe(PO4)3 and K0.97Na1.03Ti1.26Fe0.74(PO4)3 are isotypic and crystallize in the langbeinite structure type. K+ and Na+ cations, and Ti3+, Ti4+ and Fe3+ cations, respectively, share the same sites in the crystal structure.


Chemical context
Over the last decade, numerous research efforts have been directed towards the creation of new phosphate materials for Li-or Na-ion batteries (Nose et al., 2013;Zhang et al., 2021). In particular, significant progress has been made for complex phosphates with general formula M I 1+x Z 2 (PO 4 ) 3 (M I = Li, Na; Z = polyvalent metals; x values can range from 0 to 3; Zatovsky et al., 2016) adopting NASICON-type structures. The composition of phosphates with a langbeinite-type structure is very similar to the composition of NASICON-type ones, and langbeinite-type phosphates are also considered to be potential hosts for new electrode materials (Luo et al., 2019). However, langbeinite-type phosphates with a composition M I 1+x Z 2 (PO 4 ) 3 (x = 0-1) can only be prepared with large monovalent cations (e.g., K, Rb, Cs, NH 4 ; Norberg, 2002;Ogorodnyk et al., 2007a). The langbeinite-type structure has only been reported for Na 2 Z III Ti(PO 4 ) 3 (Z III = Cr, Fe; Isasi & Daidouh, 2000). More recently, a good prospect for using such kinds of materials as anodes for Na-ion batteries has been predicted because of the recently reported migration mechanisms in langbeinite-type Na 2 CrTi(PO 4 ) 3 determined by atomic simulation (Luo et al., 2019). However, according to Wang et al. (2019), the phosphate Na 2 CrTi(PO 4 ) 3 belongs to the family of compounds with a NASICON-type structure. Therefore, the issue of preparing Na-containing langbeinitetype phosphates requires further research and development. ISSN 2056-9890 In recent years, the synthesis of K/Na-containing complex phosphates has been realized using the self-flux method and resulted in the compounds K 1.75 Na 0.25 Ti 2 (PO 4 ) 3 (Zatovsky et al., 2018) and K 0.877 Na 0.48 Ti 2 (PO 4 ) 3 .

Structural commentary
As it is illustrated in Fig. 1, two pairs of mixed sites occupied by alkali metals (K/Na) and transition metals (Ti/Fe) are located on threefold rotation axes (Wyckoff position 4 a), whereas the P and all O atoms occupy general sites (12 b). In the structures, the main structural element for building of the three-dimensional framework is a [(Ti/Fe) 2 (PO 4 ) 3 ] fragment consisting of two mixed-metal [(Ti/Fe)O 6 ] octahedra and three PO 4 tetrahedra (Fig. 2a). Such building units run in three orthogonal directions along the cubic space diagonals (Fig. 2b), which is typical for the langbeinite-related family of compounds (sulfates, phosphates, vanadates etc, Ogorodnyk et al., 2007a).
Two octahedrally coordinated sites (Ti1/Fe1) and (Ti2/Fe2) show mixed occupancy with an Fe:Ti ratio close to 1:1. For (I), the Ti occupancy is 0.48 (3) for the M1 site, while for the M2 site it is 0.52 (3); for (II), the Ti occupancy is 0.61 (2) for the M1 site and 0.65 (2) for the M2 site. In the case of (I), this corresponds to Fe 3+ and Ti 4+ cations, while for (II), the simultaneous presence of Fe 3+ , Ti 3+ and Ti 4+ is suggested. The prepared crystals of (II) are violet in color and the Ti 3+ :Ti 4+ ratio is about 1:4 taking into account the total charge of the cationic part of the compound. Partial self-reduction of Ti 4+ to Ti 3+ often accompanies the synthesis of langbeinite-type complex phosphates in fluxes of multicomponent systems when various trivalent or divalent metals are present (Gustafsson et al., 2005;Zatovskii et al., 2006). For structures (I) and (II), the [Ti/FeO 6 ] octahedra are slightly distorted (Tables 1 and 2 (Ogorodnyk et al., 2008(Ogorodnyk et al., , 2007b(Ogorodnyk et al., , 2006. The P-O distances for both structures are in the narrow ranges of 1.516 (4)-1.523 (3) for (I) and 1.517 (3)-1.523 (2) Å for (II).

Synthesis and crystallization
Phosphates (I) and (II) were obtained from the melts of the system Na 2 O-K 2 O-P 2 O 5 -TiO 2 -Fe 2 O 3 at fixed molar ratios of (Na+K)/P = 1.0, Ti/P = 0.20 and different values of Na/K = 1.0 or 2.0 over the temperature range 1273-953 K. All initial components M I H 2 PO 4 (M I = Na, K), Fe 2 O 3 and TiO 2 were of an analytical grade. A mixture of KH 2 PO 4 (10 g), NaH 2 PO 4 (8.82 g), Fe 2 O 3 (2.35 g) and TiO 2 (2.35 g) was used for the preparation of (I), while a mixture of KH 2 PO 4 (10 g), NaH 2 PO 4 (17.64 g), Fe 2 O 3 (3.53 g) and TiO 2 (3.53 g) was used for the preparation of (II). In both cases, the mixtures of calculated amounts of starting components were ground in an agate mortar and melted in a platinum crucible at 1273 K. The obtained melts were kept under isothermal conditions for 2 h for dissolving of the corresponding TiO 2 + Fe 2 O 3 mixture in the phosphate melt. Then the temperature was decreased with a rate of 25 K h À1 to 953 K and kept at this temperature for 2 h before cooling down to room temperature by turning off the furnace power. The obtained crystalline phases were separated from soluble salts by leaching with hot water and dried at 373 K.
The molar ratio Na/K in the initial melts had an influence on the composition of the obtained crystals. Light-yellow crystals formed in the melt with a ratio of Na:K = 1.0 while violet crystals were obtained for a ratio Na:K = 2.0 ( Fig. 3). It should be noted that increasing the amount of sodium in the initial melts to a ratio Na/K = 2.0 caused the growth of crystals with sizes of 2-3 mm ( Fig. 3b) in length.
The chemical compositions of the prepared samples (quantitative determination of K, Na, Ti, Fe and P) were confirmed by ICP-AES with a Shimadzu ICPE-9820 spectrometer. The analyses showed that the molar ratios of K:Na:Ti:Fe:P were close to 1.65:0.35:1:1:3 for (I) and 1:1:1.25:0.75:3 for (II).
The phosphates (I) and (II) were further characterized using Fourier transform infrared (FTIR) spectroscopy. The spectra were obtained using a PerkinElmer Spectrum BX spectrometer in the range 4000-400 cm À1 (at 4 cm À1 resolution) with sample material pressed into KBr pellets. The FTIR spectra for both compounds are similar in band positions of vibration modes (Fig. 4). The broad and intense bands in the frequency region 1150-900 cm À1 are characteristic for P-O stretching vibrations [ as (PO 3 ) -region 1150-1090 cm À1 and s (PO 3 ) -region 1020-900 cm À1 ] of the PO 4 tetrahedron. The band group at 650-550 cm À1 is caused by bending (P-O) vibrations of P-O bonds. Some differences in the spectra were observed in the range 500-400 cm À1 , which are due to X-O (X = Ti, Fe) vibrations and correlate with insignificant differences in the composition of the prepared compounds (I) and (II).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. According to the results of the chemical analysis, large quantities of Na and Ti are present in the structures. Taking into account possible coordination spheres of Na and Ti and previously reported langbeinite-type phosphates with a mixed-metal framework, we supposed that Ti occupies the same sites as Fe, and Na the same positions as K. Hence, the corresponding positions of Fe1 and Fe2, K1 and K2 were occupied with Ti and Na, respectively. As the Fe(Ti) positions are part of the rigid framework, we assumed that these sites show full occupancy, while the sites related with the alkali metal can be fully or partially occupied. At a first approach, the occupancies were refined using linear combinations of free variables (SUMP restraint). Two SUMP restraints were applied to occupancies of Fe1(Ti1) and Fe2(Ti2) sites. One more SUMP restraint was then applied to the sum of valence units of all metal-atom positions. This refinement resulted in satisfactory reliability factors. It was found that the occupancies of K1(Na1) and K2(Na2) are close to 1. Thus, to simplify the refinement we tried to refine the occupancies with free variable constraints instead of SUMP restraints while keeping the alkali metal site occupancies equal to 1. To each refined position, a unique free variable constraint was applied, plus constrained identical coordinates and ADPs for each site. The resulting reliability factors were Photographs of single crystals of (a) (I) and (b) (II).
found to be almost equal to those where the SUMP restraints were used. For the final refinement cycles, the second approach was applied to both structures.

Potassium sodium titanium iron tris(phosphate) (I)
Crystal data

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (  0.0254 (7) 0.0254 (7) 0.0254 (7) 0.0004 (5) 0.0004 (5) 0.0004 (5) Na1 0.0254 (7) 0.0254 (7) 0.0254 (7)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.