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The structure of potassium nitrate, KNO3, has been redetermined at room temperature. The compound surprisingly shows a 2 × 2 × 1 superstructure and crystallizes in space group Cmc21. This result contrasts with that found in former investigations, which gave the supergroup Pmcn, neglecting the superstructure. The improved results are due to the employment of a CCD area detector.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270103025277/iz1035sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270103025277/iz1035Isup2.hkl
Contains datablock I

Comment top

Potassium nitrate is known to be both a mineral (niter) and an inexpansive chemical compound. On heating, it shows two phase transitions at atmosheric pressure. The first transition is from the α-phase to the β-phase and the second is from the β-phase to the melt. These transitions are widely used to calibrate differential thermal analysis (DTA) devices (Barshad, 1952; Greis et al., 1985). The crystal structure of the α-phase was determined by Edwards (1931), and it was redetermined by Nimmo & Lucas (1973) and Holden & Dickinson (1975) using neutrons and X-rays, respectively. All these authors found an aragonite-type structure with space group Pmnc.

For many years, we have used potassium nitrate to calibrate a DTA apparatus and to teach graduate students phase transitions observed by DTA and X-ray powder diffraction. Then, in addition, we advantageously used the same compound to teach structure determination as well. To study the structure, we employed a Weissenbrg camera and an Enraf–Nonius CAD-4 diffractometer. The Weissenberg photographs never showed a sign of a superstructure and, therefore, the known space group, Pmnc, was confirmed. However, the situation was completely different when we used a Nonius KappaCCD diffractometer. In the latter case, space group Cmc21 was determined, with doubled crystallographic a0 and b0 axes. All corresponding reflections with odd h or k indices have weak intensities, thus showing a 2x2x1 superstructure. Based on the criterion I>2σ(I), 1060 reflections are classified as significant, of which 1012 belong to the sublattice and 48 to the superlattice, including 17 Friedel pairs. The intensity of the strongest superlattice reflection is less than 2% of that of the strongest sublattice reflection.

The space group of the superstructure (Cmc21) is a subgroup of that given in the literature (Pmcn) for α-KNO3. As can be seen from the International Tables for Crystallography (1983, Vol·A), both space groups are connected via the minimal supergroup (maximal subgroup) Pmc21 (Cmc21 < Pmc21 < Pmcn). According to the work? of Lima-de-Faria et al. (1990), the superstructure of α-KNO3 and the formerly described aragonite-type structure of this compound are homeotypic. Regarding the geometric properties, the superstructure differs little from the aragonite-type structure. However, the superstructure has a lowered symmetry and hence a higher number of crystallographically independent atoms in the asymmetric unit. While the true aragonite-type structure has one unique K atom and one unique N atom, each of them sitting on a mirror plane, the superstructure contains three unique atoms of both elements. Each of these elements occupies two unique sites on a mirror plane and one at a general position, as depicted in Fig. 1.

There are seven unique O atoms in the superstructure of α-KNO3. All seven are bonded to N atoms, forming nitrate ions, of which three are unique (see Table 1 for selected bond lengths and angles). In each nitrate ion, the N atom deviates by 2 s.u. or less from the plane through the O atoms. Each of the three unique potassium ions is surrounded by nine O atoms from six nitrate ions, thereby building an irregular coordination polyhedron.

A face-indexing analytical absorption correction was applied, based on a data crystal bound by ten faces, viz. (0–11), (01–1), (010), (0–10), (110), (−1–10), (0–1-1), (−111), (−110) and (1–10), with face-to-center distances of 0.11, 0.06, 0.16, 0.17, 0.13, 0.11, 0.12, 0.15, 0.10 and 0.17 mm, respectively. Neither looking orthorhombic nor resembling a sphere, the shape of the crystal is a little strange, which makes absorption correction extremely difficult.

Refinement top

In order to obtain the maximum number of superstructure reflections, we chose a high angle, θmax, of measured reflections. Therefore, and because of the general weakness of the superstructure reflections, we unavoidably obtained many reflections below the 2σ(I) threshold, making the refinement a little clumsy. Moreover, the pseudo-symmetries imposed by the substructure result in strong correlations of several parameters. Additional correlations are possibly introduced via absorption correction errors (see above). As a result of the correlations, the convergence of the refinement is bad. The use of damping parameters was necessary in the last cycles of refinement. A final cycle with no damping was added. Because of the presence of a pseudo-centre of symmetry, the absolute structure could not be determined. The Flack parameter is 0.1, with a standard uncertainty of 0.5. Simple twinning models based on primitive monoclinic lattices neither resulted in a better convergence of the refinement nor removed the pseudo-centre of symmetry. However, if the transformation of potassium nitrate from the trigonal ferroelectric high-temperature γ-phase to the room-temperature α-phase is led by symmetry then, in? the α-phase, a C-centred cell lacking a centre of symmetry is desired. Therefore, the pseudo-symmetries may be true features of the α-phase structure.

Computing details top

Data collection: COLLECT (Nonius 1999); cell refinement: COLLECT; data reduction: DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997).

Figures top
[Figure 1] Fig. 1. A view along [001] of the α-KNO3 superstructure, showing an asymmetric unit with N—O bonds. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x,y,z; (ii) 1 − x,y,z.]
potassium nitrate top
Crystal data top
KNO3F(000) = 800
Mr = 101.11Dx = 2.101 Mg m3
Orthorhombic, Cmc21Mo Kα radiation, λ = 0.71070 Å
Hall symbol: C 2c -2Cell parameters from 2170 reflections
a = 10.825 (6) Åθ = 1.0°–40.3°°
b = 18.351 (10) ŵ = 1.46 mm1
c = 6.435 (3) ÅT = 293 K
V = 1278.3 (12) Å3Irregular, approximately isometric, colourless
Z = 160.33 × 0.25 × 0.17 mm
Data collection top
Nonius KappaCCD
diffractometer
3857 independent reflections
Radiation source: fine-focus sealed tube1069 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.000
area detector scansθmax = 40.2°, θmin = 2.2°
Absorption correction: analytical
face-indexing analytical, see text
h = 1919
Tmin = 0.65, Tmax = 0.78k = 3333
3857 measured reflectionsl = 1111
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0308P)2 + 2.4843P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.037(Δ/σ)max = 0.036
wR(F2) = 0.160Δρmax = 0.46 e Å3
S = 1.04Δρmin = 0.35 e Å3
3857 reflectionsExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
101 parametersExtinction coefficient: 0.0097 (7)
1 restraintAbsolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Primary atom site location: structure-invariant direct methodsAbsolute structure parameter: 0.1 (5)
Crystal data top
KNO3V = 1278.3 (12) Å3
Mr = 101.11Z = 16
Orthorhombic, Cmc21Mo Kα radiation
a = 10.825 (6) ŵ = 1.46 mm1
b = 18.351 (10) ÅT = 293 K
c = 6.435 (3) Å0.33 × 0.25 × 0.17 mm
Data collection top
Nonius KappaCCD
diffractometer
3857 independent reflections
Absorption correction: analytical
face-indexing analytical, see text
1069 reflections with I > 2σ(I)
Tmin = 0.65, Tmax = 0.78Rint = 0.000
3857 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0371 restraint
wR(F2) = 0.160Δρmax = 0.46 e Å3
S = 1.04Δρmin = 0.35 e Å3
3857 reflectionsAbsolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
101 parametersAbsolute structure parameter: 0.1 (5)
Special details top

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
K10.00000.41703 (12)0.2604 (3)0.0310 (6)
K20.50000.41679 (12)0.2614 (3)0.0305 (6)
K30.24980 (14)0.16670 (8)0.25104 (8)0.0303 (4)
N10.00000.2483 (6)0.4215 (19)0.026 (2)
N20.50000.2469 (6)0.4208 (19)0.031 (2)
N30.2497 (6)0.4979 (4)0.0911 (18)0.0271 (18)
O10.00000.1807 (4)0.414 (2)0.043 (3)
O20.50000.1788 (4)0.4138 (19)0.038 (2)
O30.0999 (4)0.2830 (3)0.4206 (15)0.0401 (15)
O40.3987 (4)0.2809 (3)0.4248 (15)0.0434 (18)
O50.1506 (4)0.5313 (3)0.0984 (15)0.0405 (18)
O60.3479 (4)0.5321 (3)0.0896 (16)0.044 (2)
O70.2478 (5)0.4296 (3)0.0937 (18)0.043 (2)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
K10.0274 (12)0.0276 (13)0.0381 (15)0.0000.0000.0011 (14)
K20.0286 (12)0.0278 (13)0.0352 (15)0.0000.0000.0009 (14)
K30.0262 (8)0.0280 (8)0.0366 (12)0.0000 (7)0.0004 (7)0.0008 (12)
N10.021 (4)0.030 (4)0.028 (6)0.0000.0000.000 (5)
N20.038 (5)0.036 (5)0.019 (6)0.0000.0000.002 (5)
N30.026 (3)0.031 (3)0.024 (5)0.007 (3)0.003 (3)0.001 (4)
O10.038 (5)0.031 (4)0.059 (7)0.0000.0000.001 (5)
O20.045 (5)0.025 (4)0.044 (6)0.0000.0000.002 (5)
O30.031 (3)0.047 (3)0.042 (4)0.007 (3)0.004 (3)0.005 (4)
O40.026 (3)0.045 (3)0.059 (5)0.009 (3)0.003 (3)0.010 (4)
O50.029 (3)0.036 (3)0.057 (5)0.002 (3)0.004 (3)0.001 (4)
O60.028 (3)0.037 (3)0.066 (6)0.007 (3)0.006 (3)0.002 (4)
O70.045 (4)0.032 (3)0.052 (6)0.003 (3)0.000 (3)0.011 (4)
Geometric parameters (Å, º) top
K1—O2i2.840 (11)K3—O4i2.814 (8)
K1—O5ii2.854 (7)K3—O7vii2.826 (10)
K1—O52.854 (7)K3—O3i2.832 (8)
K1—O3ii2.878 (7)K3—O42.870 (6)
K1—O32.878 (7)K3—O5viii2.881 (6)
K1—O5iii2.879 (8)K3—O6viii2.881 (7)
K1—O5iv2.879 (8)K3—O32.894 (6)
K1—O72.898 (7)K3—O12.911 (5)
K1—O7ii2.898 (7)K3—O22.912 (5)
K2—O6v2.837 (8)N1—O11.241 (12)
K2—O6iii2.837 (8)N1—O31.255 (6)
K2—O1i2.864 (11)N1—O3ii1.255 (6)
K2—O6vi2.900 (7)N2—O21.250 (12)
K2—O62.900 (7)N2—O4vi1.262 (6)
K2—O42.921 (7)N2—O41.262 (6)
K2—O4vi2.921 (7)N3—O61.235 (8)
K2—O7vi2.945 (7)N3—O51.237 (8)
K2—O72.945 (7)N3—O71.253 (9)
O2i—K1—O5ii99.7 (3)O1i—K2—O7vi76.3 (2)
O2i—K1—O599.7 (3)O6vi—K2—O7vi43.59 (16)
O5ii—K1—O569.7 (2)O6—K2—O7vi109.1 (2)
O2i—K1—O3ii75.66 (18)O4—K2—O7vi123.25 (14)
O5ii—K1—O3ii122.98 (16)O4vi—K2—O7vi81.52 (15)
O5—K1—O3ii166.82 (11)O6v—K2—O7141.6 (3)
O2i—K1—O375.66 (18)O6iii—K2—O773.05 (18)
O5ii—K1—O3166.82 (11)O1i—K2—O776.3 (2)
O5—K1—O3122.98 (16)O6vi—K2—O7109.1 (2)
O3ii—K1—O344.14 (19)O6—K2—O743.59 (16)
O2i—K1—O5iii142.86 (12)O4—K2—O781.52 (15)
O5ii—K1—O5iii110.96 (16)O4vi—K2—O7123.25 (14)
O5—K1—O5iii73.17 (12)O7vi—K2—O7135.9 (4)
O3ii—K1—O5iii102.9 (3)O4i—K3—O7vii143.24 (16)
O3—K1—O5iii78.3 (2)O4i—K3—O3i69.9 (2)
O2i—K1—O5iv142.86 (12)O7vii—K3—O3i141.67 (16)
O5ii—K1—O5iv73.17 (12)O4i—K3—O4111.22 (15)
O5—K1—O5iv110.96 (16)O7vii—K3—O498.5 (3)
O3ii—K1—O5iv78.3 (2)O3i—K3—O474.44 (17)
O3—K1—O5iv102.9 (3)O4i—K3—O5viii104.7 (3)
O5iii—K1—O5iv69.0 (3)O7vii—K3—O5viii73.93 (17)
O2i—K1—O776.0 (2)O3i—K3—O5viii79.1 (2)
O5ii—K1—O7109.6 (3)O4—K3—O5viii123.52 (15)
O5—K1—O743.72 (16)O4i—K3—O6viii79.3 (2)
O3ii—K1—O7123.27 (14)O7vii—K3—O6viii75.43 (17)
O3—K1—O781.56 (16)O3i—K3—O6viii102.7 (3)
O5iii—K1—O774.28 (19)O4—K3—O6viii166.55 (12)
O5iv—K1—O7141.0 (3)O5viii—K3—O6viii43.54 (13)
O2i—K1—O7ii76.0 (2)O4i—K3—O373.07 (17)
O5ii—K1—O7ii43.72 (16)O7vii—K3—O399.9 (3)
O5—K1—O7ii109.6 (3)O3i—K3—O3111.39 (15)
O3ii—K1—O7ii81.56 (16)O4—K3—O368.28 (15)
O3—K1—O7ii123.27 (14)O5viii—K3—O3166.94 (12)
O5iii—K1—O7ii141.0 (3)O6viii—K3—O3124.20 (15)
O5iv—K1—O7ii74.28 (19)O4i—K3—O173.0 (2)
O7—K1—O7ii135.5 (5)O7vii—K3—O177.5 (3)
O6v—K2—O6iii70.9 (3)O3i—K3—O1140.8 (3)
O6v—K2—O1i141.98 (13)O4—K3—O1108.5 (3)
O6iii—K2—O1i141.98 (13)O5viii—K3—O1123.12 (16)
O6v—K2—O6vi73.34 (11)O6viii—K3—O182.21 (18)
O6iii—K2—O6vi111.83 (16)O3—K3—O143.81 (17)
O1i—K2—O6vi99.1 (3)O4i—K3—O2140.7 (3)
O6v—K2—O6111.83 (16)O7vii—K3—O276.0 (3)
O6iii—K2—O673.34 (11)O3i—K3—O273.2 (2)
O1i—K2—O699.1 (3)O4—K3—O244.11 (16)
O6vi—K2—O669.2 (2)O5viii—K3—O280.81 (17)
O6v—K2—O4103.4 (3)O6viii—K3—O2122.45 (15)
O6iii—K2—O478.3 (2)O3—K3—O2109.3 (2)
O1i—K2—O475.38 (19)O1—K3—O2136.7 (4)
O6vi—K2—O4166.79 (11)O1—N1—O3120.4 (5)
O6—K2—O4123.20 (16)O1—N1—O3ii120.4 (5)
O6v—K2—O4vi78.3 (2)O3—N1—O3ii119.2 (10)
O6iii—K2—O4vi103.4 (3)O2—N2—O4vi119.8 (5)
O1i—K2—O4vi75.38 (19)O2—N2—O4119.8 (5)
O6vi—K2—O4vi123.19 (16)O4vi—N2—O4120.4 (10)
O6—K2—O4vi166.79 (11)O6—N3—O5119.8 (7)
O4—K2—O4vi44.09 (19)O6—N3—O7121.5 (6)
O6v—K2—O7vi73.05 (18)O5—N3—O7118.6 (6)
O6iii—K2—O7vi141.6 (3)
Symmetry codes: (i) x+1/2, y+1/2, z1/2; (ii) x, y, z; (iii) x, y+1, z+1/2; (iv) x, y+1, z+1/2; (v) x+1, y+1, z+1/2; (vi) x+1, y, z; (vii) x+1/2, y+1/2, z+1/2; (viii) x+1/2, y1/2, z.

Experimental details

Crystal data
Chemical formulaKNO3
Mr101.11
Crystal system, space groupOrthorhombic, Cmc21
Temperature (K)293
a, b, c (Å)10.825 (6), 18.351 (10), 6.435 (3)
V3)1278.3 (12)
Z16
Radiation typeMo Kα
µ (mm1)1.46
Crystal size (mm)0.33 × 0.25 × 0.17
Data collection
DiffractometerNonius KappaCCD
diffractometer
Absorption correctionAnalytical
face-indexing analytical, see text
Tmin, Tmax0.65, 0.78
No. of measured, independent and
observed [I > 2σ(I)] reflections
3857, 3857, 1069
Rint0.000
(sin θ/λ)max1)0.909
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.037, 0.160, 1.04
No. of reflections3857
No. of parameters101
No. of restraints1
Δρmax, Δρmin (e Å3)0.46, 0.35
Absolute structureFlack H D (1983), Acta Cryst. A39, 876-881
Absolute structure parameter0.1 (5)

Computer programs: COLLECT (Nonius 1999), COLLECT, DENZO (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997).

Selected geometric parameters (Å, º) top
N1—O11.241 (12)N2—O41.262 (6)
N1—O31.255 (6)N3—O61.235 (8)
N1—O3i1.255 (6)N3—O51.237 (8)
N2—O21.250 (12)N3—O71.253 (9)
N2—O4ii1.262 (6)
O1—N1—O3120.4 (5)O4ii—N2—O4120.4 (10)
O1—N1—O3i120.4 (5)O6—N3—O5119.8 (7)
O3—N1—O3i119.2 (10)O6—N3—O7121.5 (6)
O2—N2—O4ii119.8 (5)O5—N3—O7118.6 (6)
O2—N2—O4119.8 (5)
Symmetry codes: (i) x, y, z; (ii) x+1, y, z.
 

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