Growth and single-crystal refinement of phase-III potassium nitrate, KNO₃

Oriented single crystals of the high-temperature phase of KNO₃ (phase III), a ferroelectric compound that may also occur as an atmospheric aerosol particle, were grown at room temperature and pressure by atomizing a solution of KNO₃ in water and allowing droplets to dry on a glass substrate. The crystals are up to 1 mm across and are stable unless mechanically disturbed. There is no evidence of the spontaneous transformation of phase III to the room-temperature stable phase (phase II), even after several months. Single-crystal structure determinations of phase III were obtained at 295 and 123 K. The unit cell regained its room-temperature dimensions after warming from 123 K. The phase-III KNO₃ structure can be viewed as the stacking parallel to the c axis of alternating K atoms and planar NO₃ groups. The NO₃ groups connect the planes of K atoms, where each O is fourfold coordinated to one N and three K. Each K atom has nine O nearest neighbors, with three bonds at 2.813 and six at 2.9092 Å. The interatomic K-N-K distance alternates from 5.051 to 3.941 along the c axis. The N-O distances increase from 1.245 (2) Å at 295 K to 1.2533 (15) Å at 123 K. The nitrate group has a slight non-planarity, with the N atoms 0.011 Å above the O plane and directed toward the more distant K of the K-N-K chain.

Computing details

For both compounds, data collection: SMART V5.632; cell refinement: SAINT V6.45A; data reduction: SAINT V6.45A; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL Version 6.14 (Sheldrick, 2003b); software used to prepare material for publication: SHELXTL Version 6.14 (Sheldrick, 2003b).

(rt)

Crystal data

KNO3	Dx = 2.162 Mg m^−3
Mr = 101.11	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, R3m	Cell parameters from 715 reflections
Hall symbol: R 3 -2"	θ = 4.9–27.4°
a = 5.4698 (8) Å	µ = 1.50 mm^−1
c = 8.992 (3) Å	T = 298 K
V = 232.99 (8) Å^3	Plate, colorless
Z = 3	0.11 × 0.08 × 0.02 mm
F(000) = 150

Data collection

Bruker SMART APEX diffractometer	157 independent reflections
Radiation source: fine-focus sealed tube	157 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.024
ω scan	θmax = 27.4°, θmin = 4.9°
Absorption correction: multi-scan SADABS Version 2.10 (Sheldrick, 2003a)	h = −7→7
Tmin = 0.858, Tmax = 0.966	k = −7→7
764 measured reflections	l = −11→11

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0348P)^2 + 0.0102P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.022	(Δ/σ)max < 0.001
wR(F^2) = 0.054	Δρmax = 0.13 e Å^−3
S = 1.23	Δρmin = −0.16 e Å^−3
157 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
13 parameters	Extinction coefficient: 0.157 (16)
0 restraints	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.03 (8)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
K1	0.0000	0.0000	0.5000	0.0357 (4)
N1	0.6667	0.3333	0.3950 (4)	0.0312 (8)
O1	0.5352 (2)	0.0705 (4)	0.3961 (2)	0.0467 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K1	0.0298 (4)	0.0298 (4)	0.0475 (5)	0.01488 (19)	0.000	0.000
N1	0.0296 (12)	0.0296 (12)	0.0345 (13)	0.0148 (6)	0.000	0.000
O1	0.0403 (10)	0.0267 (9)	0.0685 (13)	0.0133 (5)	0.0002 (4)	0.0003 (8)

Geometric parameters (Å, º)

K1—O1^i	2.813 (2)	K1—K1^v	4.3540 (7)
K1—O1^ii	2.814 (2)	N1—O1^vi	1.245 (2)
K1—O1^iii	2.9092 (9)	O1—K1^vii	2.9092 (8)
K1—N1^iv	3.2962 (11)
O1^i—K1—O1^ii	72.14 (7)	O1^i—K1—K1^v	138.713 (16)
O1^i—K1—O1^iii	145.89 (7)	O1^ii—K1—K1^v	138.712 (15)
O1^ii—K1—O1^iii	99.06 (4)	O1^viii—K1—K1^v	90.67 (5)
O1^viii—K1—O1^iii	73.80 (2)	O1^iii—K1—K1^v	39.65 (4)
O1^iii—K1—O1^ix	43.52 (8)	O1^ix—K1—K1^v	72.29 (4)
O1^iii—K1—O1	140.13 (8)	O1—K1—K1^v	114.27 (4)
O1^ix—K1—O1	110.20 (4)	N1^iv—K1—K1^v	56.99 (4)
O1^iii—K1—O1^x	69.42 (8)	N1^iii—K1—K1^v	57.00 (4)
O1^ix—K1—O1^x	110.21 (4)	O1^vi—N1—O1	119.993 (6)
O1^i—K1—O1^xi	73.798 (19)	O1^vi—N1—O1^xii	119.994 (6)
O1^i—K1—N1^iv	83.37 (5)	O1—N1—O1^xii	119.992 (6)
O1^ii—K1—N1^iv	149.48 (8)	O1^vi—N1—K1^vii	162.9 (2)
O1^iii—K1—N1^iv	91.03 (4)	O1—N1—K1^vii	61.22 (5)
O1^ix—K1—N1^iv	129.28 (6)	K1^vii—N1—K1^xiii	112.14 (5)
O1^x—K1—N1^iv	22.03 (4)	N1—O1—K1^xiv	132.4 (2)
O1^xi—K1—N1^iv	22.04 (4)	N1—O1—K1	96.74 (7)
O1^ii—K1—N1^iii	83.36 (5)	K1^xiv—O1—K1	99.06 (4)
N1^iv—K1—N1^iii	112.14 (5)	K1—O1—K1^vii	140.13 (8)

Symmetry codes: (i) −x+y+2/3, −x+1/3, z+1/3; (ii) x−1/3, y+1/3, z+1/3; (iii) x−1, y, z; (iv) x−1, y−1, z; (v) x−2/3, y−1/3, z−1/3; (vi) −x+y+1, −x+1, z; (vii) x+1, y, z; (viii) −y−1/3, x−y−2/3, z+1/3; (ix) −y, x−y, z; (x) −x+y, −x, z; (xi) −y, x−y−1, z; (xii) −y+1, x−y, z; (xiii) x+1, y+1, z; (xiv) x+1/3, y−1/3, z−1/3.

(123K)

Crystal data

KNO3	Dx = 2.233 Mg m^−3
Mr = 101.11	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, R3m	Cell parameters from 750 reflections
Hall symbol: R 3 -2"	θ = 4.9–27.5°
a = 5.4325 (2) Å	µ = 1.55 mm^−1
c = 8.8255 (7) Å	T = 123 K
V = 225.56 (2) Å^3	Plate, colorless
Z = 3	0.11 × 0.08 × 0.02 mm
F(000) = 150

Data collection

Bruker SMART APEX diffractometer	155 independent reflections
Radiation source: fine-focus sealed tube	155 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.014
ω scan	θmax = 27.5°, θmin = 4.9°
Absorption correction: multi-scan SADABS Version 2.10 (Sheldrick, 2003a)	h = −7→7
Tmin = 0.854, Tmax = 0.965	k = −7→7
755 measured reflections	l = −11→11

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.017	w = 1/[σ^2(Fo^2) + (0.0254P)^2 + 0.214P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.041	(Δ/σ)max < 0.001
S = 1.14	Δρmax = 0.18 e Å^−3
155 reflections	Δρmin = −0.19 e Å^−3
12 parameters	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
0 restraints	Absolute structure parameter: −0.05 (7)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
K1	0.0000	0.0000	0.5000	0.0153 (2)
N1	0.6667	0.3333	0.3975 (3)	0.0133 (6)
O1	0.53347 (16)	0.0669 (3)	0.39836 (16)	0.0185 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K1	0.0130 (2)	0.0130 (2)	0.0200 (3)	0.00650 (12)	0.000	0.000
N1	0.0128 (9)	0.0128 (9)	0.0142 (11)	0.0064 (4)	0.000	0.000
O1	0.0163 (7)	0.0101 (7)	0.0271 (8)	0.0051 (3)	−0.0001 (3)	−0.0003 (5)

Geometric parameters (Å, º)

K1—O1^i	2.7799 (14)	K1—K1^iv	4.3002 (2)
K1—O1^ii	2.8778 (5)	N1—O1^v	1.2533 (15)
K1—N1^iii	3.2643 (9)	O1—K1^vi	2.7798 (14)
O1^i—K1—O1^vii	71.84 (5)	O1^i—K1—K1^iv	138.615 (9)
O1^i—K1—O1^ii	98.93 (3)	O1^vii—K1—K1^iv	138.614 (9)
O1^vii—K1—O1^ii	145.09 (5)	O1^viii—K1—K1^iv	90.52 (3)
O1^viii—K1—O1^ii	73.299 (11)	O1^ii—K1—K1^iv	39.69 (3)
O1^ii—K1—O1^ix	44.32 (6)	O1^ix—K1—K1^iv	72.96 (3)
O1^ii—K1—O1^x	141.42 (5)	O1^x—K1—K1^iv	115.02 (3)
O1^ix—K1—O1^x	110.75 (3)	N1^iii—K1—K1^iv	57.32 (3)
O1^vii—K1—O1^xi	98.92 (3)	O1^v—N1—O1	119.996 (4)
O1^viii—K1—O1^xi	73.298 (11)	O1^v—N1—K1^xii	61.18 (4)
O1^ii—K1—O1^xi	69.04 (6)	O1^x—N1—K1^xii	163.6 (2)
O1^i—K1—N1^iii	83.01 (4)	K1^xii—N1—K1^xiii	112.63 (4)
O1^vii—K1—N1^iii	148.73 (6)	N1—O1—K1^vi	132.30 (16)
O1^ii—K1—N1^iii	22.43 (3)	N1—O1—K1	96.39 (6)
O1^x—K1—N1^iii	130.28 (5)	K1^vi—O1—K1	98.93 (3)
O1^xi—K1—N1^iii	91.08 (3)	K1—O1—K1^xii	141.42 (5)
N1^iii—K1—N1^xi	112.63 (5)

Symmetry codes: (i) −x+y+2/3, −x+1/3, z+1/3; (ii) −x+y, −x, z; (iii) x−1, y−1, z; (iv) x−2/3, y−1/3, z−1/3; (v) −y+1, x−y, z; (vi) x+1/3, y−1/3, z−1/3; (vii) x−1/3, y+1/3, z+1/3; (viii) −y−1/3, x−y−2/3, z+1/3; (ix) −y, x−y−1, z; (x) −x+y+1, −x+1, z; (xi) x−1, y, z; (xii) x+1, y, z; (xiii) x+1, y+1, z.