Crystal structure of lutetium aluminate (LUAM), Lu4Al2O9

Single-crystal X-ray structure analysis revealed that Lu4Al2O9 is isostructural with Eu4Al2O9 and contains Lu atoms in six- and sevenfold coordination, together with tetrahedral Al atoms.

The crystal structure of the title compound containing lutetium, the last element in the lanthanide series, was determined using a single crystal prepared by heating a pressed pellet of a 2:1 molar ratio mixture of Lu 2 O 3 and Al 2 O 3 powders in an Ar atmosphere at 2173 K for 4 h. Lu 4 Al 2 O 9 is isostructural with Eu 4 Al 2 O 9 and composed of Al 2 O 7 ditetrahedra and Lu-centered six-and sevenfold oxygen polyhedra. The unit-cell volume, 787.3 (3) Å 3 , is the smallest among the volumes of the rare-earth (RE) aluminates, RE 4 Al 2 O 9 . The crystal studied was refined as a two-component pseudo-merohedric twin.

Chemical context
In the Al 2 O 3 -Lu 2 O 3 system, where Lu has the largest atomic number among the rare-earth elements (RE), the following three phases have been reported: Lu 3 Al 5 O 12 , LuAlO 3 , and Lu 4 Al 2 O 9 . These phases have been actively investigated as host materials, not only for phosphors (Ding et al., 2011;Xiang et al., 2016;Wang et al., 2018), but also for scintillators, owing to their large radiation absorption cross sections arising from the presence of Lu. Various scintillation properties of Ce-and Pr-doped Lu 3 Al 5 O 12 and LuAlO 3 crystals have been characterized (Wojtowicz, 1999;Nikl, 2000;Wojtowicz et al., 2006;Nikl et al., 2013), and the luminescence properties of Ce-and Pr-doped Lu 4 Al 2 O 9 evaluated (Lempicki et al., 1996;Zhang et al., 1997, Zhang et al., 1998Drozdowski et al., 2005). The crystal structures of the lutetium aluminates Lu 3 Al 5 O 12 (Euler & Bruce, 1965) and LuAlO 3 (Dernier & Maines, 1971;Shishido et al., 1995) have been determined as garnet-type (LUAG) and perovskite-type (LUAP), respectively. However, to date, there have been no reports of the lattice constants of Lu 4 Al 2 O 9 , although Shirvinskaya & Popova (1977) treated it as isotypic with Y 4 Al 2 O 9 and have reported the d-spacings and relative peak intensities in the powder X-ray diffraction pattern (PDF#00-033-0844).

Structural commentary
X-ray diffraction spots from the Lu 4 Al 2 O 9 single crystal were indexed on the basis of a monoclinic unit cell with lattice parameters: a = 7.236 (2) Å , b = 10.333 (2) Å , c = 11.096 (3) Å , and = 108.38 (2) . As shown in Fig. 1, the unit-cell volume of Lu 4 Al 2 O 9 calculated from these parameters lies on the extrapolated line of RE 4 Al 2 O 9 volumes plotted against the effective ionic radii for sixfold coordination of the trivalent rare-earth anions (RE 3+ ) (Shannon, 1976). In other words, Lu 4 Al 2 O 9 containing Lu, which has the smallest effective ionic radius of the RE atoms, has the smallest unit-cell volume in the RE 4 Al 2 O 9 series, in line with predictions arising from the lanthanide contraction.
The crystal structure of Lu 4 Al 2 O 9 (space group P2 1 /c), determined using Eu 4 Al 2 O 9 (Brandle & Steinfink, 1969) as the starting model, contains two crystallographically distinct Al sites, four Lu sites, and nine O sites. The two Al sites are tetrahedrally coordinated by oxygen atoms. The two Al tetrahedra are connected through a shared O5 atom, forming an Al 2 O 7 ditetrahedral oxy-aluminate group (Fig. 2). The Al 2 O 7 dimers lie parallel to the a axis, and are related by the c glide symmetry operation (Fig. 3) Unit-cell volume of RE 4 Al 2 O 9 versus effective ionic radius for the trivalent rare-earth anion (RE 3+ ) with sixfold coordination.

Synthesis and crystallization
The starting powders Al 2 O 3 (Sumitomo Chemicals, AKP20, 99.99%) and Lu 2 O 3 (Nippon Yttrium, 99.999%) were mixed in a molar ratio of Lu:Al = 2:1, ground with ethanol in an agate mortar, and pressed into a pellet. The pellet was placed in a BN crucible with an inner diameter of 18 mm and a height of 20 mm. The BN crucible was covered with a BN lid, and heated in a chamber with a carbon heater (Shimadzu Mectem, Inc., VESTA). The pellet was heated slowly under vacuum ($10 À2 Pa) from room temperature to 1273 K. During the 5 min. hold at 1273 K, the chamber was filled with Ar (99.9995%) up to 0.15 MPa. The temperature was then raised to 2173 K at a heating rate of 300 Kh À1 . After being held at Acta Cryst. (2020). E76, 752-755 research communications Figure 3 The crystal structure of Lu 4 Al 2 O 9 highlighting the Al 2 O 7 ditetrahedra viewed down the b axis (upper), and the Al 2 O 7 ditetrahedra and Lucentered polyhedra viewed down the a axis (lower). Steinfink, 1969) were replaced by Lu atoms to generate the initial model. Several iterations of refinement yielded an R value of 10.07% and a residual electron density of $10 e Å À3 . A subsequent refinement, performed by implementing the (100) twin plane observed in a study of Y 4 Al 2 O 9 (Yamane et al., 1995b), yielded an R(F 2 > 2(F 2 )) value of 1.92% with an approximate volume ratio of 6:4 for the twin domains.  program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).

Lutetium aluminate
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.