Understanding of the structural chemistry in the uranium oxo-tellurium system under HT/HP conditions

The study of phase formation in the U-Te-O systems with mono and divalent cations under high-temperature high-pressure (HT/HP) conditions has resulted in four new inorganic compounds: K2 [(UO2) (Te2O7)], Mg [(UO2) (TeO3)2], Sr [(UO2) (TeO3)2] and Sr [(UO2) (TeO5)]. Tellurium occurs as TeIV, TeV, and TeVI in these phases which demonstrate the high chemical flexibility of the system. Uranium VI) adopts a variety of coordinations, namely, UO6 in K2 [(UO2) (Te2O7), UO7 in Mg [(UO2) (TeO3)2] and Sr [(UO2) (TeO3)2], and UO8 in Sr [(UO2) (TeO5)]. The structure of K2 [(UO2) (Te2O7)] is featured with one dimensional (1D) [Te2O7]4- chains along the c-axis. The Te2O7 chains are further linked by UO6 polyhedra, forming the 3D [(UO2) (Te2O7)]2- anionic frameworks. In Mg [(UO2) (TeO3)2], TeO4 disphenoids share common corners with each other resulting in infinite 1D chains of [(TeO3)2]4- propagating along the a-axis. These chains link the uranyl bipyramids by edge sharing along two edges of the disphenoids, resulting in the 2D layered structure of [(UO2) (Te2O6)]2-. The structure of Sr [(UO2) (TeO3)2] is based on 1D chains of [(UO2) (TeO3)2]∞ 2− propagating into the c-axis. These chains are formed by edge-sharing uranyl bipyramids which are additionally fused together by two TeO4 disphenoids, which also share two edges. The 3D framework structure of Sr [(UO2) (TeO5)] is composed of 1D [TeO5]4− chains sharing edges with UO7 bipyramids. Three tunnels based on 6-Membered rings (MRs) are propagating along [001] [010] and [100] directions. The HT/HP synthetic conditions for the preparation of single crystalline samples and their structural aspects are discussed in this work.


Introduction
The structural and chemical diversity of oxo-tellurium and uranium bearing phases has attracted researchers in the field of solid state and materials chemistry, especially in regard to the diverse oxidation states and coordination geometries that tellurium and uranium can adopt in oxo-based phases (Christy et al., 2016). In nature, a number of minerals within this family have been reported: cliffordite [(UO 2 ) (Te 3 O 7 )] (Brandstatter, 1981), moctezumite (PbUO 2 (TeO 3 ) 2 ) (Swihart et al., 1993), schmitterite (UO 2 (TeO 3 ) (Meunier and Galy, 1973), and more recently markcooperite (Pb 2 (UO 2 )TeO 6 ) (Kampf et al., 2010). Besides the attention due to the diverse fundamental aspects of inorganic chemistry, the presence of tellurium within spent nuclear fuel and its corrosive capabilities also makes this thematic relevant for environmental issues (Kleykamp, 1985). Moctezumite and schmitterite, for example, are secondary minerals commonly observed in telluride-bearing ores (Swihart et al., 1993).
The predominant oxidation state of uranium in oxidizing conditions is U VI . Hereby, uranium is present as almost linear trans dioxo-cations (UO 2 2+ , the so called uranyl group) both in solid state as well as in solution. Within the solid state, the typical coordination environment surrounding the dioxo-cations is a bipyramid, in which the uranyl forms the central axis and additional four to six oxygen atoms occupy sites within the equatorial plane, leading to tetragonal, pentagonal or hexagonal bipyramids (Burns et al., 1997;Hao et al., 2016;Hao et al., 2017a;Hao et al., 2017b;Hao et al., 2018;Hao et al., 2020a;Hao et al., 2022).
Tellurium is typically present as Te IV or Te VI and hereby in form of oxo-anions, tellurites and tellurates, respectively. Te IV is known to adopt several coordination environments, for example, pyramidal TeO 3 , disphenoidal TeO 4 and square pyramidal TeO 5 (Balraj and Vidyasagar, 1999;Kim et al., 2007a;Kim et al., 2010). A further inter-connection via corner-sharing can lead to complex oxotellurium polymers (Lindqvist and Moret, 1973;Hafidi et al., 1986;Mao et al., 2008;Lin et al., 2013). In some compounds several coordination environments of Te can be found, for example, NH 4 ATe 4 O 9 ·2H 2 O (A = Rb, Cs) in which all three coordination geometries of Te are present (Kim and Halasyamani, 2008). Additionally, the lone electron pairs in Te IV have a strong influence on the diversity of Te-O coordination environments. The hexavalent Te (Te VI ) typically adopts trigonal bipyramidal, distorted octahedral or tetrahedral coordination in oxygen phases. A few ditellurates contain mixed valent tellurium, Te IV and Te VI , ACuTe 2 O 7 (A = Sr, Ba, or Pb) and BaMTe 2 O 7 (M = Mg or Zn) have been previously reported (Yeon et al., 2011;Yeon et al., 2012). Compared to Te IV or Te VI , which are more stable at ambient conditions, inorganic Te V bearing phases were scarcely reported (Lindqvist and Moret, 1973;Hafidi et al., 1986;Balraj and Vidyasagar, 1999;Kim et al., 2007a;Mao et al., 2008;Kim et al., 2010;Lin et al., 2013).
The different structural units have a pronounced influence on the dimensionality of the resulting phases. In the presence of hexavalent uranium with uranyl groups, typically two-dimensional structures dominate (Burns et al., 1996;Burns et al., 1997). This is a direct consequence of oxo-anions typically only being able to condense perpendicular to the terminal uranyl group of the uranyl polyhedra (Burns et al., 1997;Hao et al., 2016;Hao et al., 2017a;Hao et al., 2017b;Hao et al., 2018;Hao et al., 2022). However, less than half of the currently known phases in uranyl oxo-tellurium system (15 of 32 found in the ICSD (Bergerhoff et al., 1987)), crystallize as twodimensional structures. In this atypical formation of many one-and three-dimensional structures, the presence of the stereochemically active lone pair, plays a central role (Almond and Albrecht-Schmitt, 2002;Xiao et al., 2016a).
From a materials science point-of-view, studies on tellurium-based phases have been focused on the synthesis of non-centrosymmetric phases (NCS). These phases are of interest for potential applications in the fields of second harmonic generation (SHG) as well as ferro-and piezo-and pyroelectricity (Almond and Albrecht-Schmitt, 2002;Xiao et al., 2016a). This acentric behavior is also addressed by the presence of the aforementioned stereochemically active lone pairs present in Te IV . This has resulted in rich results of NCS crystal structures in recent years (Chi et al., 2006;Kim et al., 2014). However, the presence of acentric tellurite groups does not necessarily need to result in NCS phases. The acentric units can order themselves to counteract a potential global NCS structure (Chi et al., 2006;Kim et al., 2007b;Kim et al., 2014).
We have recently systematically studied the A-U-Te-O (A = alkali and alkaline Earth metal) system under extreme conditions (HT/HP). Our goal is to further understand the different chemical behavior of actinide-tellurium oxo-phases from extreme conditions compared to conventional ones and to develop a methodology of how these phases crystallize under the extreme environment. As a result of this study, a series of quaternary oxide tellurium materials, K 2 [(UO 2 ) (Te 2 O 7 )], Mg [(UO 2 ) (TeO 3 ) 2 ], Sr [(UO 2 ) (TeO 3 ) 2 ] and Sr [(UO 2 ) (TeO 5 )] have been prepared by the HT/HP solid state reaction method. In which, K 2 [(UO 2 ) (Te 2 O 7 )] is a very rare example of a Te V bearing phase. The detailed HT/HP synthetic routes, high-temperature and high-pressure behavior, and topology of the structures are discussed.

Experimental section
Caution! The UO 2 (NO 3 ) 2 ·6H 2 O used in this work contained natural uranium; nevertheless the standard precautions for handling radioactive materials must be followed. The γ-UO 3 was formed simply by heating the uranyl nitrate and analyzing it with powder XRD (Engmann and De Wolff, 1963) for its purity as we always use the γ-UO 3 as the initial compound in the HT/HP synthesis.

Crystal growth
All the titled compounds were synthesized in the form of small single crystals using the high-temperature/high-pressure solid-state method. All the chemicals were obtained from commercial sources as analytically pure and used without further purification.
K 2 [(UO 2 ) (Te 2 O 7 )]. Uranium trioxide UO 3 (20.0 mg, 0.0699 mmol), KNO 3 (21.2 mg, 0.208 mmol), TeO 2 (22.3 mg, 0.140 mmol), and H 6 TeO 6 (64.2 mg, 0.279 mmol) in a molar ratio of UO 3 : KNO 3 : TeO 2 : H 6 TeO 6 = 1 : 3: 2 : 4 were mixed together and finely ground. Then, the mixture was filled into a platinum capsule (outer diameter: 4 mm, wall thickness: 0.2 mm, length:7 mm). The capsule was sealed on both sides with an impulse micro welding device (Lampert PUK U4) and placed into the center of a 1/2-inch piston cylinder talc-pyrex assembly. After this, the capsule was inserted into a 6 mm diameter MgO spacer and positioned in the center of a tapered graphite furnace. The final run pressure of 3.5 GPa was applied within 30 min, then the temperature program was started. With a heating rate of 100 K min -1 the temperature was increased to the maximum temperature of 1173 K. After 1 h of annealing, the temperature was decreased to 570 K over a time period of 106 h (cooling rate 0.11 K min -1 ). At 570 K, the experiment was automatically quenched to room temperature. After decompression for 20 min, the capsule was extracted out of the high-pressure assembly and broken. The product of yellow , UO 3 (20.0 mg, 0.0699 mmol), Mg(NO 3 ) 2 (20.7 mg, 0.140 mmol), TeO 2 (33.5 mg, 0.211 mmol), and H 6 TeO 6 (16.1 mg, 0.070 mmol) were weighed with a molar ratio of 1:3:1:2 and subsequently thoroughly ground before being filled into platinum capsule. The operations of sealing the platinum capsule and opening it after the reactions are same as mentioned above for the synthesis of K 2 [(UO 2 ) (Te 2 O 7 )]. The pressure of 3.5 GPa was used within 30 min, then the temperature program was started. It was heated up to 1373 K with a heating rate of 100 K min -1 . After a holding time of 4 h at 1373 K, the temperature was decreased to 1173 K in 1h, and then cooled to 623 K over a time period of 90 h (cooling rate 0.10 K min -1 ). At 623 K, the experiment was automatically quenched to room temperature. After decompression for 20 min, the capsule was extracted out of the high-pressure assembly and broken. The product containing small yellow crystals were picked up for further analysis.
Sr[(UO 2 ) (TeO 3 ) 2 ] and Sr[(UO 2 ) (TeO 5 )]. Both phases coprecipitated using a finely ground mixture of UO 3 (30.0 mg, 0.105 mmol), Sr(CO 3 ) (15.5 mg, 0.105 mmol), TeO 2 (16.7 mg, 0.105 mmol), and H 6 TeO 6 (24.1 mg, 0.0.105 mmol) with a molar ratio of 1:1:1:1. The pressure of 3.5 GPa was applied within 30 min, then the temperature program was started. It was heated up to 1273 K with a heating rate of 100 K min -1 . After a holding time of 4 h at 1273 K, the temperature was cooled down to 1073 K in 1h, and then decreased to 573 K over a time period of 50 h (cooling rate 0.17 K min -1 ). At 573 K, the experiment was automatically quenched to room temperature. After decompression for 20 min, the capsule was extracted out of the high-pressure assembly and broken. The product containing small yellow crystals together with colorless remains of the educts, mainly consisting of SrCO 3 . The small yellow crystals were picked up for further analysis.

Crystallographic studies
Single crystal X-ray diffraction data for all four compounds were collected on an Agilent Technologies SuperNova diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All data sets were corrected for Lorentz and polarization factors as well as for absorption by the multi-scan method (Sheldrick, 1998). The structures of all four compounds were solved by the direct method and refined by a full-matrix least-squares fitting on F 2 by SHELX (Almond and Albrecht-Schmitt, 2002;Xiao et al., 2016a). Their structures were checked for possible missing symmetry elements using PLATON with the ADDSYM algorithm, and no higher symmetry was found (Spek, 2001). Crystallographic data and structural refinements for all compounds are summarized in Table 1

Bond-valence analysis
As a semi-empirical method for the approximate determination of valence states, BVS of all atoms in both phases were calculated.

Results and discussion
3.1 Crystal growth )] co-precipitated using a finely ground mixture of UO 3 , Sr(CO 3 ), TeO 2 and H 6 TeO 6 with a molar ratio of 1:1:1:1. We presumed that the ratios of TeO 2 and H 6 TeO 6 were used in the original reagents has also played a key role for the final oxidation states of tellurium in the compounds. The obtained materials have been found in the form of relatively small single crystals (up to 1 mm size). We presume that they grow up within so called self-flux which is usual for the materials crystallization from multicomponent high-temperature solid-state systems (Balraj and Vidyasagar, 1999;Kim et al., 2007a;Kim et al., 2010).
There is one crystallographically unique tellurium site Te (1), one uranium site U (1), one potassium site K (1) and five oxygen sites O (1)-O (5) in the structures of K 2 [(UO 2 ) (Te 2 O 7 )], respectively. Corner-shared TeO 6 octahedra form a onedimensional double-chain along the c-axis, and these chains are further connected by UO 6 polyhedra along the b-axis, resulting in the 3D network ( Figure 1E). The UO 6 pseudooctahedral coordination alternate between the Te-O double chains and are corner-shared (O5) to two TeO 6 octahedra and edge-shared (O2-O3) to two TeO 6 octahedra. The Te-O bond distances in TeO 6 octahedra range between 1.888(4) and 1.979 (4) Supplementary Table S1. These bond lengths are comparable with previously reported works Frontiers in Chemistry frontiersin.org (Almond and Albrecht-Schmitt, 2002;Chi et al., 2006;Kim et al., 2007b;Kim et al., 2014;Xiao et al., 2016a are isostructural, and their two-dimensional crystal structure consists of 2D layers based upon corner-sharing CuO 5 square pyramids, TeO 6 octahedra, and TeO 4 dispheniods. BaMTe 2 O 7 (M = Mg 2+ and Zn 2+ ) are iso-structural with BaCuTe 2 O 7 , and exhibit a crystal structure composed of layers of corner-shared MO 5 (M = Mg 2+ or Zn 2+ ) square pyramids, TeO 6 octahedra, and TeO 4 polyhedra. The [MTe 2 O 7 ] 2− anionic layers (M = Mg 2+ and Zn 2+ ) stack along the b-axis, and are separated by Ba 2+ cations. Comparing these phases we can presume that high-pressure conditions applied in our study not only condensed the final structures (from 2D towards 3D), but also influenced the redox stability of Te with stabilization of the rare Te V cation.

Structure of Mg[(UO 2 ) (TeO 3 ) 2 ]
Mg [(UO 2 ) (TeO 3 ) 2 ] crystallizes in the orthorhombic space group Cmca. Mg, U and Te occupy one crystallographically independent position each and for oxygen three crystallographically independent positions are occupied. As shown in Figure 4, Uranium is eight fold-coordinated by oxygen in bipyramidal fashion. The two oxygen positions located at the pyramid tops have a U-O bond length of 1.805(6) Å. Together with a bond angle of 180.0°between O-U-O along the central axis, these are typical values for a uranyl group. The equatorial oxygen atoms adopt bond-distances from 2.361(7) to 2.527(5) Å.
Tellurium is coordinated by four oxygen atoms resulting in disphenoidal symmetry. The electron lone pair points towards the top center of the disphenoid. The bond distances range from 1.881(4) to 2.042(3) Å. This coordination is found to be common within Te IV structures (Balraj and Vidyasagar, 1999;Xiao et al., 2016b).
In Mg [(UO 2 ) (TeO 3 ) 2 ], TeO 4 disphenoids share common corners with each other resulting in infinite onedimensional  Figure 4B). Within these sheets, four UO 8 hexagonal bipyramids and four TeO 4 disphenoids form a ring structure. This is well visible in the topologic description shown in Figure 4C. Mg 2+ are located within the rings of UO 8 and TeO 4 mentioned above. It is coordinated by six oxygen atoms in distances ranging from 2.051(5) to 2.284(6) Å leading to a rectangular bipyramid. Mg 2+ acts as a counter charge for the [(UO 2 ) (Te 2 O 6 )] 2− layersresulting in a neutrally charged phase. Bond valence sum calculations for all positions (Mg~2.03, U~5.98, Te~4.06) yield values in accordance to the assumed oxidation states.

Structure of Sr[(UO 2 ) (TeO 3 ) 2 ]
Sr [(UO 2 ) (TeO 3 ) 2 ] crystallizes in the orthorhombic space group Pbam. U, Sr and Te each are present on one independent position. As U and Sr lie on special positions (0.5, 0.5, 0.5 and 0.5, 0.0, 0.5, respectively) and Te lies on a general position in respect to x and y (0.2894, 0.2644, 0.0), the multiplicity results in a U:Te molar ratio of 1:2. Oxygen is present on four independent positions, each of them are general positions. The structure of Sr [(UO 2 ) (TeO 3 ) 2 ] is depicted in Figure 5A.
Uranium is coordinated by two short-bonded actinyl oxygen with <U-O yl > = 1.825 (13) Å and six equatorial oxygen (<U-O eq > = 2.349(7) Å) forming hexagonal bipyramids and thus hexavalent U is present. BVS calculations are well in agreement with 6.21 v. u. Te is coordinated by four oxygen atoms forming a disphenoidal coordination polyhedron. This coordination is typical for tetravalent Te and BVS calculations yield 4.01 v. u., supporting the assignment for tetravalent Te. The bonding distances range from 1.849 (13) to 2.102(4) Å. Eight oxygen positions surround Sr with bond distances of 2.525(7) to 3.029 (14) Å. Sr is predominantly stable as divalent Sr and a BVS of 2.09 v. u. is well in agreement with this.
The structure is based on one-dimensional chains of [(UO 2 ) (TeO 3 ) 2 ] 2− propagating into the [001] direction. These chains are formed by edge-sharing uranyl bipyramids which are additionally fused together by two TeO 4 disphenoids, edge-sharing along two edges. Such a chain is depicted in Figure 5B. The according topology is shown in Figure 5C  Frontiers in Chemistry frontiersin.org by two TeO 4 disphenoids, which also share two edges. We can see that with the counter cations radii increasing from Mg 2+ to Sr 2+ this results in the structure of materials changing from a 2D to a 1D structural type. In this case we can speak of a morphotropic transition within A II [(UO 2 ) (TeO 3 ) 2 ] (A II -alkali-earth elements).

Structure of Sr[(UO 2 ) (TeO 5 )]
Sr [(UO 2 ) (TeO 5 )] crystallizes in the orthorhombic space group Pbam. The crystallographic data is given in Table 1. It forms a 3D framework structure made up of UO 7 and TeO 6 polyhedra with Sr 2+ cations filling the voids to achieve charge neutrality.
Two uranium positions are present within the structure and both have a typical bipyramidal pentagonal oxygen coordination of UO 7 . The bond lengths of the uranyl oxygen positions are < U1-O yl > = 1.81598) Å and <U2-O yl > = 1.822(7) Å. This can be explained by the dense packing of the framework leading to a stronger coordination of the equatorial plane with average bond lengths of <U1-O eq > = 2.353(7) Å and <U2-O eq > = 2.357(1) Å, respectively. The closer coordination of the equatorial oxygen positions is charge compensated by the uranyl bonds. Both, U1O 7 and U2O 7 , are not interconnected to each other, are however interlinked by octahedral TeO 6 units. The resulting framework is described in more detail below.
Te only adopts a single position and is coordinated by six oxygen atoms to form distorted octahedral polyhedra (Supplementary Figure  S1). Hereby, two positions are slightly elongated (O1 and O4) with Te-O1 = 1.975(2) Å and Te-O4 = 1.980(2) Å. The other four positions range from 1.8805) Å to 1.9802) Å. Two TeO 6 polyhedra are interconnected by corner sharing of the O1 and O4 positions, leading to one-dimensional chains propagating in the [001] direction.
Three crystallographically independent Sr positions are present in Sr [(UO 2 ) (TeO 5 )]. All three positions are shown in Figures 6A-C. Sr1 is located within [100] channels and coordinated tenfold by oxygen with distances ranging from 2.460(5) Å to 3.175(7) Å. Sr2 and Sr3 are eightfold coordinated with distances ranging from 2.429(5) Å to 2.985(9) Å and 2.454(5) Å to 2.935(5) Å, respectively. Sr2 is located within [010] channels and Sr3 within [001] channels. To describe the framework structure of Sr [(UO 2 ) (TeO 5 )], it is best to divide the structure into simpler one and two-dimensional units. This is shown in Figure 7. As already stated above, the structure is made up of infinite [TeO 5 ] 4− chains. Two opposite edges of the TeO 6 octahedron are involved in edge-sharing with U2O 7 and U1O 7 bipyramids and the two remaining oxygen positions each corner-share with a U1O 7 and a U2O 7 (Supplementary Figure S1). The same framework topology can be found in Na 2 [(UO 2 ) (TeO 5 )] (Almond and Albrecht-Schmitt, 2002;Xiao et al., 2016a).