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KPrF4, potassium praseodymium tetrafluoride, is isotypic with β-KCeF4 and KSmF4. The structure is composed of a three-dimensional framework of [PrF9] and [KF7] polyhedra whose corresponding coordination geometries might be described as a tricapped and a monocapped trigonal prism, respectively. The [KF7] polyhedra form buckled 2[KF1/1F2/2F4/4] layers parallel to (001), which are connected perpendicularly by the intermediate Pr atoms along [001].

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536803005178/br6085sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S1600536803005178/br6085Isup2.hkl
Contains datablock I

Key indicators

  • Single-crystal X-ray study
  • T = 293 K
  • Mean [sigma](Pr-F) = 0.002 Å
  • R factor = 0.018
  • wR factor = 0.048
  • Data-to-parameter ratio = 16.2

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

General Notes

ABSTM_02 The ratio of expected to reported Tmax/Tmin(RR') is < 0.90 Tmin and Tmax reported: 0.187 0.755 Tmin' and Tmax expected: 0.192 0.652 RR' = 0.842 Please check that your absorption correction is appropriate.

Comment top

Host lattices based on complex alkaline earth fluorides are promising materials for luminescence applications when they are doped or substituted with rare earth elements (Nakajima et al., 2000). The previously described phase KMgPr3F12 (Labeau et al., 1972) seems to be an appropriate candidate for that purpose. Since no structure or crystallographic data have been published for this compound, crystal growth experiments were started. During these investigations the only crystals suitable for subsequent structure analyses were those of KPrF4. This phase was examined for the first time as a result of melting diagram determinations (Dergunov, 1952). Later, the magnetic properties were also investigated (Bukhalova et al., 1968) and the crystal symmetry reported `to be lower than cubic' (Bukhalova et al., 1969). The correct orthorhombic symmetry was found independently by Metin et al. (1981) and Samiev et al. (1980) who showed that the title compound is isotypic with β-KCeF4 (Brunton, 1969) and KSmF4 (Saf'yanov et al., 1973). Although during a more detailed investigation on hydrothermal synthesis and thermal behaviour lattice parameters were determined (Khaidukov et al., 1991), no structural details of KPrF4 were given. Results of a single-crystal structure refinement are reported in this article.

The unit-cell volume of the title compound (370 Å3) lies between the corresponding Ce (373 Å3) and Sm (362 Å3) phases which is caused by the contraction of the lanthanide's radius. The structure is made up from nine-coordinated Pr atoms and seven-coordinated K atoms. The polyhedron around the lanthanide atom might be described as a slightly distorted tricapped triangular prism, with an average Pr—F distance of 2.444 Å (Fig. 1, right). The alkali metal atom is surrounded by seven F atoms which form a monocapped trigonal prism with a mean K—F distance of 2.690 Å (Fig. 1, left). Two F1 atoms with a considerably longer bond length of 3.310 (2) Å augment this coordination polyhedron, but show only very weak interaction. This is supported by bond-valence calculations (Brown, 2002). Using the parameters provided by Brese & O'Keeffe (1991) leads to a bond-valence contribution of only 0.028 bvu (bond valence units) which indicates that this contribution can be disregarded.

The [KF7] polyhedra share common faces and edges to form buckled 2[KF1/1F2/2F4/4] layers parallel to (001) and a repeating unit of c/2, as emphasized in Fig. 2. The Pr atoms are located in the voids between the layers and connect the K—F sheets in a perpendicular manner along the [001] direction.

The coordination number (CN) of the F atoms are in the range 3–5 showing the following coordination geometries with slight distortions for each polyhedron: F1 (CN = 3), vertex of a trigonal pyramid; F2 and F3 (CN = 4), tetrahedron; F5 (CN = 5), square pyramid.

Experimental top

59 mg KF (Merck, p·A.), 16 mg MgF2 (Merck, Patinal) and 150 mg PrF3 [obtained by reacting Pr2O3 (Fluka, puriss.) with NH4F·HF (Fluka, p·A.)], corresponding to a molar ratio of 4:1:3, were heated in a graphite crucible under static atmsphere of a (98/2)% mixture of N2/H2 up to 1273 K in the course of 5 h. This temperature was held for 2 h and then decreased to 773 K within 50 h. After cooling to room temperature, the solidified melt was leached with demineralized water. From the remaining residue, light-green plates of KPrF4 could be isolated. Besides KPrF4 as the main phase, perovskite-type KMgF3 (de Vries & Roy, 1953) could be identified by qualitative phase analysis of the bulk sample.

Computing details top

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA implemented in PLATON (Spek, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. The coordination around potassium (left) and praseodymium (right), both with displacement ellipsoids drawn at the 75% probability level. The congruent faces which connect the polyhedra along [100] are outlined for the left figure.
[Figure 2] Fig. 2. : View of KPrF4 along [010]; the unit cell and the [KF7] polyhedra are outlined
Potassium praseodymium tetrafluoride top
Crystal data top
KPrF4F(000) = 456
Mr = 256.01Dx = 4.601 Mg m3
Orthorhombic, PnmaMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2nCell parameters from 25 reflections
a = 6.2727 (8) Åθ = 12.7–19.0°
b = 3.7821 (5) ŵ = 14.24 mm1
c = 15.578 (3) ÅT = 293 K
V = 369.58 (9) Å3Plate, light green
Z = 40.11 × 0.11 × 0.03 mm
Data collection top
Enraf-Nonius CAD-4
diffractometer
519 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.048
Graphite monochromatorθmax = 30.0°, θmin = 2.6°
ω/2θ scansh = 88
Absorption correction: numerical
(HABITUS; Herrendorf, 1993-1997)
k = 55
Tmin = 0.187, Tmax = 0.755l = 2021
4028 measured reflections3 standard reflections every 500 min
617 independent reflections intensity decay: none
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.018 w = 1/[σ2(Fo2) + (0.0265P)2 + 0.0964P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.048(Δ/σ)max = 0.001
S = 1.13Δρmax = 0.96 e Å3
617 reflectionsΔρmin = 1.49 e Å3
38 parametersExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.0074 (6)
Crystal data top
KPrF4V = 369.58 (9) Å3
Mr = 256.01Z = 4
Orthorhombic, PnmaMo Kα radiation
a = 6.2727 (8) ŵ = 14.24 mm1
b = 3.7821 (5) ÅT = 293 K
c = 15.578 (3) Å0.11 × 0.11 × 0.03 mm
Data collection top
Enraf-Nonius CAD-4
diffractometer
519 reflections with I > 2σ(I)
Absorption correction: numerical
(HABITUS; Herrendorf, 1993-1997)
Rint = 0.048
Tmin = 0.187, Tmax = 0.7553 standard reflections every 500 min
4028 measured reflections intensity decay: none
617 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.01838 parameters
wR(F2) = 0.0480 restraints
S = 1.13Δρmax = 0.96 e Å3
617 reflectionsΔρmin = 1.49 e Å3
Special details top

Experimental. The crystal shape was optimized by minimizing the R-value of selected ψ-scanned reflections using the program HABITUS (Herrendorf, 1993–97). The habit so derived was used for the numerical absorption correction.

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

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
K0.27642 (13)0.75000.20181 (6)0.01499 (18)
Pr0.25113 (2)0.25000.438200 (12)0.00661 (12)
F10.1374 (3)0.25000.03669 (14)0.0128 (4)
F20.0044 (3)0.75000.44107 (12)0.0102 (4)
F30.3956 (3)0.75000.36516 (15)0.0113 (4)
F40.0883 (3)0.25000.30555 (15)0.0143 (4)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
K0.0110 (3)0.0213 (4)0.0126 (4)0.0000.0006 (3)0.000
Pr0.00646 (14)0.00644 (15)0.00693 (16)0.0000.00008 (5)0.000
F10.0106 (9)0.0131 (9)0.0147 (12)0.0000.0026 (9)0.000
F20.0099 (9)0.0097 (10)0.0109 (11)0.0000.0005 (7)0.000
F30.0124 (9)0.0106 (9)0.0110 (10)0.0000.0026 (7)0.000
F40.0170 (9)0.0158 (10)0.0101 (10)0.0000.0026 (8)0.000
Geometric parameters (Å, º) top
K—F3i2.607 (2)Pr—F42.305 (2)
K—F2ii2.616 (2)Pr—F3v2.3857 (14)
K—F32.652 (3)Pr—F32.3857 (14)
K—F4iii2.7231 (15)Pr—F2vi2.436 (2)
K—F4ii2.7231 (15)Pr—F1ii2.4541 (19)
K—F42.7534 (17)Pr—F2v2.4795 (14)
K—F4iv2.7534 (17)Pr—F22.4795 (14)
K—F13.310 (2)Pr—F1vii2.5336 (15)
K—F1iv3.310 (2)Pr—F1viii2.5336 (15)
F3i—K—F2ii98.11 (8)F4—Pr—F3v74.97 (6)
F3i—K—F3129.96 (8)F4—Pr—F374.97 (6)
F2ii—K—F3131.93 (7)F3v—Pr—F3104.87 (9)
F3i—K—F4iii129.90 (5)F4—Pr—F2vi114.25 (7)
F2ii—K—F4iii65.59 (6)F3v—Pr—F2vi127.56 (4)
F3—K—F4iii80.68 (7)F3—Pr—F2vi127.56 (4)
F3i—K—F4ii129.90 (5)F4—Pr—F1ii125.48 (7)
F2ii—K—F4ii65.59 (6)F3v—Pr—F1ii72.61 (6)
F3—K—F4ii80.68 (7)F3—Pr—F1ii72.61 (6)
F4iii—K—F4ii87.97 (6)F2vi—Pr—F1ii120.27 (7)
F3i—K—F480.91 (6)F4—Pr—F2v74.31 (5)
F2ii—K—F4136.44 (4)F3v—Pr—F2v69.48 (6)
F3—K—F463.74 (6)F3—Pr—F2v149.17 (7)
F4iii—K—F4144.05 (5)F2vi—Pr—F2v64.87 (5)
F4ii—K—F481.70 (4)F1ii—Pr—F2v129.45 (4)
F3i—K—F4iv80.91 (6)F4—Pr—F274.31 (5)
F2ii—K—F4iv136.44 (4)F3v—Pr—F2149.17 (7)
F3—K—F4iv63.74 (6)F3—Pr—F269.48 (6)
F4iii—K—F4iv81.70 (4)F2vi—Pr—F264.87 (5)
F4ii—K—F4iv144.05 (5)F1ii—Pr—F2129.45 (4)
F4—K—F4iv86.76 (7)F2v—Pr—F299.40 (7)
F3i—K—F156.46 (5)F4—Pr—F1vii131.70 (4)
F2ii—K—F158.48 (5)F3v—Pr—F1vii140.86 (7)
F3—K—F1145.08 (2)F3—Pr—F1vii65.94 (6)
F4iii—K—F1123.60 (6)F2vi—Pr—F1vii73.01 (6)
F4ii—K—F176.10 (5)F1ii—Pr—F1vii68.34 (6)
F4—K—F187.18 (5)F2v—Pr—F1vii137.46 (6)
F4iv—K—F1137.36 (6)F2—Pr—F1vii66.32 (6)
F3i—K—F1iv56.46 (5)F4—Pr—F1viii131.70 (4)
F2ii—K—F1iv58.48 (5)F3v—Pr—F1viii65.94 (6)
F3—K—F1iv145.08 (3)F3—Pr—F1viii140.86 (7)
F4iii—K—F1iv76.10 (5)F2vi—Pr—F1viii73.01 (5)
F4ii—K—F1iv123.60 (6)F1ii—Pr—F1viii68.34 (6)
F4—K—F1iv137.36 (6)F2v—Pr—F1viii66.32 (6)
F4iv—K—F1iv87.18 (5)F2—Pr—F1viii137.46 (6)
F1—K—F1iv69.69 (5)F1vii—Pr—F1viii96.55 (7)
Symmetry codes: (i) x1/2, y, z+1/2; (ii) x+1/2, y, z+1/2; (iii) x+1/2, y+1, z+1/2; (iv) x, y+1, z; (v) x, y1, z; (vi) x, y+1, z+1; (vii) x+1/2, y+1, z+1/2; (viii) x+1/2, y, z+1/2.

Experimental details

Crystal data
Chemical formulaKPrF4
Mr256.01
Crystal system, space groupOrthorhombic, Pnma
Temperature (K)293
a, b, c (Å)6.2727 (8), 3.7821 (5), 15.578 (3)
V3)369.58 (9)
Z4
Radiation typeMo Kα
µ (mm1)14.24
Crystal size (mm)0.11 × 0.11 × 0.03
Data collection
DiffractometerEnraf-Nonius CAD-4
diffractometer
Absorption correctionNumerical
(HABITUS; Herrendorf, 1993-1997)
Tmin, Tmax0.187, 0.755
No. of measured, independent and
observed [I > 2σ(I)] reflections
4028, 617, 519
Rint0.048
(sin θ/λ)max1)0.703
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.018, 0.048, 1.13
No. of reflections617
No. of parameters38
Δρmax, Δρmin (e Å3)0.96, 1.49

Computer programs: CAD-4 Software (Enraf-Nonius, 1989), CAD-4 Software, HELENA implemented in PLATON (Spek, 2002), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), ATOMS (Dowty, 2000), SHELXL97.

Selected bond lengths (Å) top
K—F3i2.607 (2)Pr—F3v2.3857 (14)
K—F2ii2.616 (2)Pr—F32.3857 (14)
K—F32.652 (3)Pr—F2vi2.436 (2)
K—F4iii2.7231 (15)Pr—F1ii2.4541 (19)
K—F4ii2.7231 (15)Pr—F2v2.4795 (14)
K—F42.7534 (17)Pr—F22.4795 (14)
K—F4iv2.7534 (17)Pr—F1vii2.5336 (15)
Pr—F42.305 (2)Pr—F1viii2.5336 (15)
Symmetry codes: (i) x1/2, y, z+1/2; (ii) x+1/2, y, z+1/2; (iii) x+1/2, y+1, z+1/2; (iv) x, y+1, z; (v) x, y1, z; (vi) x, y+1, z+1; (vii) x+1/2, y+1, z+1/2; (viii) x+1/2, y, z+1/2.
 

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