New Layered Oxide-Fluoride Perovskites : KNaNbOF 5 and KNa

KNaNbOF5 and KNaMO2F4 (M = Mo , W), three new layered oxide-fluoride perovskites with the general formula ABB’X 6, form from the combination of a secondorder Jahn-Teller d 0 transition metal and an alkali metal (Na ) on the B-site. Alternating layers of cation vacancies and K + cations on the A-site complete the structure. The K cations are found in the A-site layer where the flu oride ions are located. The A-site is vacant in the adjacent A-site layer where the axial oxides are located. This unusual layered arrangement of unoccupied A-sites and under bonded oxygen has not been observed previously although many perovskite-related structu res are known.


Introduction
Perovskites with the general formula ABX 3 are some of the most studied structures owing to their many interesting properties including magnetism, ionic conductivity, and ferroelectricity [1][2][3].Their structure and properties can be altered by substitution at either the A-or B-sites or creating either cation or anion vacancies.For example, ferromagnetism has been achieved with the complex perovskite AA' 3 B 4 O 12 , such as CaCu 3 Ge 4 O 12 [1], and ionic conductivity has been observed for the A-site deficient perovskites Li 3x La (2/3)-x □ 1/3-2x TiO 3 (0 < x < 0.16) [2].

OPEN ACCESS
A-site cation ordering is typically seen in anion deficient perovskites, such as the antiferromagnetic YBaFe 2 O 5 where the A-site cations (Y 3+ and Ba 2+ ) are ordered owing to the oxygen vacancies (Figure 1a) [4].Ordered layers of occupied and unoccupied A-sites occur in non-stoichiometric perovskites A 1-x BO 3 [5][6][7][8].For example, La 1/3 NbO 3 contains a ReO 3 framework of NbO 6 octahedra with unoccupied A-site layers and A-site layers occupied with 2/3 La alternating along c (Figure 1b) [6,7] Examples of A-site cation ordering in stoichiometric AA'B 2 X 6 and AA'BB'X 6 perovskites with neither anion vacancies nor A-site vacancies are rare [9].Woodward et al. showed that in NaLnMnWO 6 (Ln = Ce, Pr, Sm, Gd, Dy and Ho) and NaLnMgWO 6 (Ln = Ce, Pr, Sm, Gd, Tb, Dy and Ho) the A-site cations Na and Ln prefer to order in a layered arrangement [10,11].The layered A-site ordering is stabilized by the second order Jahn Teller displacement of the d 0 transition metal W at the B-site towards the oxide that is under bonded to alleviate the lattice strain arising from the A-site cations size mismatch (Figure 1c).[4], (b) non-stoichiometric La 1/3 NbO 3 [6,7] and (c) stoichiometric NaLaMnWO 6 [10,11].
In comparison, B-site cation ordering in A 2 BB'X 6 double perovskites has been well documented [12][13][14].The majority of the B and B' cations crystallize in an ordered rock-salt structure.In general, a disordered arrangement is observed when the oxidation states of B and B' differ by less than two, whereas a difference greater than two nearly always produces an ordered arrangement [12].When the difference in oxidation state is exactly two, disordered, partially ordered or fully ordered arrangements can result, depending on the differences in size and/or bonding preference of the B and B' cations [12,15,16].
Oxyfluoride perovskites that contain ordered oxide and fluoride ions with the general formula [MO x F 6-x ] n-(x = 1, n = 2, and M = V 5+ , Nb 5+ , Ta 5+ ; x = 2, n = 2, and M = Mo 6+ , W 6+ ; x = 2, n = 3, and M = V 5+ , Nb 5+ , Ta 5+ ; x = 3, n = 3, and M = Mo 6+ ) are known but not common [17].One well known example Na 3 MoO 3 F 3 , which crystallizes in the cryolite structure and has ordered oxide and fluoride ligands, displays ferroelectric-ferroelastic properties [3].In this paper, we report a new class of A-site deficient oxyfluoride perovskites ABMO x F 6-x (where A = K + ; B = Na + ; x = 1, M = Nb 5+ ; x = 2, M = Mo 6+ , W 6+ ) that crystallize with both A-and B-site ordered cations.KNaNbOF 5 [18] and KNaMO 2 F 4 (M = Mo 6+ , W 6+ ) have a rock-salt ordering of the B-site Na and Nb/M cations.The A-sites are arranged in a layered manner where one A-site layer contains all of the K + cations and in the adjacent layer the A-sites are completely vacant.

Structure description
The polyhedral representations of KNaNbOF 5 and KNaMO 2 F 4 (M = Mo 6+ , W 6+ ) are shown in Figure 2.These three-dimensional structures can be described as an ordered rock-salt double perovskite with the general formula ABB'X 6 with ordered A-site layers where K + occupies half the A-sites and Na + and Nb 5+ or M 6+ occupy the B and B' sites, respectively.X represents one O 2-and five F -in KNaNbOF 5 and two O 2-and four F -in KNaMO 2 F 4 .Note that throughout the following discussion M corresponds to either molybdenum or tungsten atoms.
In KNaNbOF 5 , the Na + and Nb 5+ ions occupy ordered corner-shared octahedra (no tilt, a 0 a 0 a 0 ) as in the ReO 3 structure type.The Nb 5+ ions form short Nb=O bonds and one long Nb-F bond trans to the oxide (Table 1) resulting in a distorted octahedron, as seen in previously reported d 0 transition metal anionic units [19,20].Mid-IR confirms the presence of the Nb=O bond with a peak at ν s 975 cm -1 [21].The Nb-O-Na and Nb-F2-Na bond angles are 180° along the c-axis (Table 1), consistent with corner sharing octahedra with no tilt.The Nb-F3-Na bond angle is 162.50(4)°.
In KNaMO 2 F 4 , the oxide ion and its trans fluoride are ordered.The second oxide ion is disordered among the other three fluorides in the equatorial positions.Although this partial ordering occurs between the oxide and fluoride ions, the A-and B-site cations are both ordered.The B-site cations, Na + and M 6+ , form corner-sharing octahedra (no tilt, a 0 a 0 a 0 ).The M 6+ ions form one short M=O and one long M-F bond trans to the oxide (Table 1) and, as a result, distort toward the corners of the octahedra as opposed to the edges [22,23].Mid-IR confirms the presence of the M=O bond with peaks at ν s 969 cm -1 and ν as 916 cm -1 for Mo and ν s 991 cm -1 and ν as 923 cm -1 for W [21]. Again, the M-O-Na and M-F2-Na bond angles are 180° along the c-axis consistent with corner-shared octahedra with no tilt.The M-X3-Na bond angle is 164.03(5)° and 162.11(8)° for Mo and W, respectively.
The A-site K + cations occupy half of the positions in an ordered array.Along the c-axis, the K + cations are 12-coordinated to the F -in the A-site layer.In the adjacent layer, where the axial oxides are located, the A-site is vacant.The unique ABB'X 6 structure can be described as repeating AX layers of KF, BX 2 layers of Na 1/2 Nb 1/2 F 4/2 , X layers of O, BX 2 layers of Na 1/2 Nb 1/2 F 4/2 , etc (Figure 2a) for KNaNbOF 5 .KNaMO 2 F 4 can be described as repeating AX layers of KF, BX 2 layers of Na 1/2 M 1/2 (O/F) 4/2 , X layers of O, BX 2 layers of Na 1/2 M 1/2 (O/F) 4/2 , etc. (Figure 2b).

Bond valence and anionic connectivity
Electronic potentials and chemical hardness cooperatively influence the long-range order of the [NbOF 5 ] 2-anionic unit [20,23].Oxide and fluoride order can be understood in the context of Pauling's second crystal rule (PSCR) which states that anions with the largest negative potentials will occupy sites having the largest positive potentials [24].The assessment of the positive potential in the crystal frameworks, carried out by calculating the PSCR (bond strength) sum [25] and bond valence sum [24,26] around each anionic position, should match as closely as possible to the assessment of the negative potentials of each oxide and fluoride ion (Tables 2 and 3).The central Nb 5+ ion has been excluded from the PCSR calculations because we are only interested in the oxide and fluoride interactions with the extended bond network.
In a purely inorganic bond network, the oxide and fluoride ligands in the [NbOF 5 ] 2-anionic unit with the largest residual negative charge will make the most and strongest contacts to the coordination environment [20].In KNaNbOF 5 , the F2 fluorides, which occupy positions in contact with one sixcoordinate Na + and four twelve-coordinate K + (1/6 + 4 × 1/12 = 0.5 PSCR sum and bond valence sums of 0.46 vu), retain the most negative potentials (0.58 vu) (Table 3).The four remaining fluorides, F3, have less negative potentials (0.29 vu).They are each three-coordinate (one Na + and two K + ) with lower bond valence sums of 0.40 vu and PSCR sums of 0.33.
] experimental bond valence of bond "ij", where R 0 = constant dependent on i and j bonded elements, and B = 0.37; R 0 (Nb-O) = 1.911,R 0 (Nb-F) = 1.87 ; R 0 (Na-O) = 1.803,R 0 (Na-F) = 1.677; s ij = theoretical bond valence of bond "ij", calculated by solving the network equations based on the methods described by Brown [27,28]; V i , V j = experimental valences of anions "i" and cations "j".; z i , z j = charge or formal valence of anions "i" and cations "j".The observation that the O1 site in KNaNbOF 5 makes one cationic contact to Na + in the extended bond network is understandable when other crystal chemistry factors are considered.The polarizability of the oxide (6.49Å 3 ) and fluoride (1.57Å 3 ) ions [29] also influences their interactions with the extended bond network.For instance, the "harder" (relative to oxide) F2 trans fluoride interacts with one Na + (2.3501(14) Å) and four K + (2.9586(9) Å) cations (Tables 2 and 3).Although there are fewer Na-F bonds compared to K-F, the Na-F interactions are much stronger, as seen by their shorter bond lengths and higher bond valences.Thus, the Na + cation is significantly over bonded (1.22 vu) with respect to K + (1.08 vu).The K + cation is softer than Na + , therefore it needs to make more contacts to the fluorides in order to satisfy PSCR.At the same time, the Nb 5+ ion displaces away from the trans fluoride toward the "softer" oxide which has the lowest PSCR sum and bond valence sum of 0.17 and 0.26, respectively.As a result, the oxide attracts one Na + in the extended bond network at 2.2961(16) Å and is under bonded.

Centrosymmetry versus Noncentrosymmetry
KNaNbOF 5 has a noncentrosymmetric polymorph [20].The results discussed here on the noncentrosymmetric polymorph can be found in [20].The different local coordination environments around the centrosymmetric (CS) and noncentrosymmetric (NCS) [NbOF 5 ] 2-structures are illustrated in Figure 3.In the NCS polymorph the two fluorides F1 and F3 contain the largest residual negative charge (0.48 and 0.23 vu, respectively) and make three contacts with two eight-coordinate K + and one six-coordinate Na + .In the CS polymorph, one fluoride, F1, has the largest residual negative charge (0.58 vu) and makes four contacts with four twelve-coordinate K + and one six-coordinate Na + .Although there are fewer Na-F interactions than K-F in both the NCS and CS polymorphs, the Na-F interactions are stronger, as seen by their shorter bond lengths and higher bond valences.As a result, in the CS polymorph the Na + cation is slightly more over bonded (1.22 vu) compared to K + (1.08 vu).The oxides in both the NCS and CS polymorph are under bonded.In the NCS polymorph, the oxide is three-coordinate with one K + and one Na + contact and a Nb=O bond at 1.745(5) Å.In the CS polymorph, the oxide is two-coordinate and makes one Na + contact and as expected there is a shorter Nb=O bond distance of 1.7179 (14) Å.Conversely, the extra K + contact in the NCS polymorph results in a longer Nb=O bond.
In a perovskite with B-site rock-salt ordering the A-site cations prefer to order in a layered arrangement which is stabilized by the presence of a second order Jahn Teller displacement of at least one d 0 transition metal at the B-site [10,11].In NaLnMnWO 6 and NaLnMgWO 6 with a trivalent and monovalent A-site cation, the oxides coordinated to the Na + are under bonded while the oxides coordinated to the Ln 3+ are over bonded.To compensate, the d 0 transition metal, W 6+ , at the B-site will shift towards the oxide coordinated to the smaller cation (Na + ) and away from the larger cation (Ln 3+ ) to alleviate the lattice strain arising from the size mismatch (Figure 4a).A similar comparison can be made to KNaNbOF 5 .Instead of having two A-site cations, there is an occupied and unoccupied A-site.As a result, the oxide in the same layer as the unoccupied A-site is under bonded.To compensate, the Nb 5+ at the B-site shifts toward the oxide and away from the trans fluoride (Figure 4b).Thus, the long range ordering of the KNaNbOF 5 structure results in a centrosymmetric arrangement of the anionic units.
Figure 4.The d 0 transition metal (blue) at the B-site shifts towards the oxide that contains the smallest A-site cation, Na (grey) for (a) NaLaMnWO 6 [10,11] or the vacancy in (b) KNaNbOF 5 .

KNaMO 2 F 4 and partial anionic ordering
In KNaMO 2 F 4 (M = Mo 6+ , W 6+ ), the oxide and its trans fluoride are ordered while the second oxide is disordered along with the other three fluorides at the equatorial positions.Despite the partial ordering of the oxide and fluoride, the A-and B-site cations are ordered.The partial ordering of the anionic unit has been reported previously, where Mo/W distorts towards the corner as opposed to the edge [22,23].The partial ordering of the oxide and fluoride is due to the symmetric environment around the equatorial positions.In KNaNbOF 5 , the bond network at the oxide differs from the trans fluoride; therefore, the oxide can be identified based on bond lengths.When two oxides are present as in KNaMO 2 F 4 , one oxide is ordered (Figure 5).The bond lengths of the oxide and the trans fluoride at the axial positions are different, which we use to identify the oxide and fluoride sites.However, at the equatorial positions, the bond length is the average of the four bonds from the second oxide and three fluorides, therefore the second oxide and three fluorides are not differentiated.In addition, bond valence sums cannot be accurately calculated because it is dependent on the observed bond lengths.

Materials and synthesis
Caution: Hydrofluoric acid is toxic and corrosive, and must be handled with extreme caution and the appropriate protective gear!If contact with the liquid or vapor occurs, proper treatment procedures should immediately be followed [30][31][32].

Conclusions
The interactions of the [NbOF 5 ] 2-and [MO 2 F 4 ] 2-anionic units with the cationic bond network (Na + and K + ) influences the long range ordering of the anionic units.The oxide coordinated to the unoccupied A-sites is under bonded.To compensate, the d 0 transition metal (Nb/M) shifts towards the oxide coordinated to the unoccupied A-site and away from the fluoride coordinated to the K + to satisfy PSCR.These interactions in KNaNbOF 5 and KNaMO 2 F 4 lead to a rock salt ordering of the B-site cations and an ordered layering of the occupied and unoccupied A-sites resulting in a centrosymmetric arrangement.

Table 3 .
Estimations of the negative potentials of the ions of the (NbOF 5 ) 2-anions and the surrounding positive potentials in KNaNbOF 5 , through bond strength and bond valence calculations.
a Anionic BV (Bond Valence) = z i -S Nb-O/F , where z i is the electric charge of each ligand and S Nb-O/F is taken from the Table2for each Nb-O/F bond; b Cationic PSCR sum = ∑ ∑ j,cat is the electric charge of each A i alkali cation bonded to a given ligand and v j,cat is its coordination number; c Cationic BV sum = ∑S, where S i,cat is taken from Table2for each A i -O/F bond.