Neutron structure of strontium pentacyanonitrosylferrate(II) tetrahydrate below the 153 K phase transition

Eur. J. Solid State lnorg. Chem.

t. 35, 1998,p. 667-678

Neutron structure of strontium pentacyanonitrosylferrate(H) tetrahydrate below the 153 K phase transition G. CHEVRIER Laboratoire L6on-Brillouin, CEN Saclay, 91191 Gif-sur-Yvette cedex, France A. NAVAZA* Laboratoire de Physique, Centre Pharmaceutique, 92290 Ch~tenay-Malabrycede×, France J.-M. KIAT Laboratoire L6on-Brillouin, CEN Sac]ay, 91191 Gif-sur-Yvette cedex, France, Laboratoire de Chimie-Physique du Solide, URA CNRS 453, l~cole Centrale de Paris, 92295 Chfitenay-Malabrycedex, France J.A. G[)IDA CEQU1NOR (Cent~ode QufmicaInorg~nica),Departementode Quimica,Facultadde Ciencias Exactas, UniversidadNacional de La Plata, Calle 47 y 115, C.C. 962, 1900 La Plata, R. Argentina ; Departementode CienciasB~ic~s,UniversidadNacionalde Lujfin,rutas5 y 7, 6700 Luj~, R. Argentina (P. H., received October 23, 1998; accepted October 26, 1998.) ABSTRACT. - The average structure in the space group C2/m at 130 K of strontium pentacyanonitrosylferrate(II) tetrahydrate (strontium nitroprusside tetrahydrate, Sr[Fe(CN)sNO].4H20, space group P2/m, monoclinic, Z--4, a=19.816(24), b=7.563(7), c=8.406(9) A, 13=99.02°, V=1244(24) A3) has been determined using neutron diffraction. A final R factor 0.061 was obtained using 665 observed structure factors. When going from room temperature to 130 K the space group C2/m changes to P2/m. A rearrangement of hydrogen atoms is produced due to 14.3, 17.2, 5.9 and 26 ° rotations of the planes of Wl, W2, W3 and W4 water molecules, respectively. As a consequence of this structural modification, the weakening of some hydrogen bonds related to the W4 molecule and the reinforcement of the hydrogen bonds of the W2 molecule along the [0 1 0] direction are observed.

~TRODUCTION The interest on strontium nitroprusside tetrahydrate (SrNP) derives from several physical properties: SrNP contains the nitroprusside a n i o n in which long-living metastable states are produced by laser irradiation at low temperatures [1]. Three hydrates o f SrNP are known: i.e. tetra-, di- and m o n o h y d r a t e *Author to whom correspondenceshould be addressed Eur. J. Solid State lnorg. Chem., 0992-4361/98/10-1 l/~ Elsevier, Paris

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G. CHEVRIERet aL

[2-8], where the water molecules are weakly bonded. SrNP.4H20 crystals undergo several phase transitions when going from room temperature to 77 K [3,4]. - The crystal packing of SrNP.4H20 is an excellent system to study correlation effects (Davydov splitting) due to strongly polar optic modes and also the coupling of IR radiation to longitudinal optic (LO) vibrations associated with these modes [9]. Several crystallographic and spectroscopic studies on SrNP.4H20 have been performed using X-ray powder diffraction [2-4], X-ray and neutron diffraction with single crystals [7-9], IRS [2,4,8-10], H-NMRS [11] and DTA [4]. The structural behavior for SrNP.4H20 between room temperature and 77 K seems to depend on the way the sample is thermally handled. DTA experiments reveal five transition points at 152.7+3.3 K (exothermic), 174.3_+0.4 K, 183.0_+0.5 K, 193.0+1.0 K and 199.5_+0.6 K (all endothermic, not always reproducible) if the sample is first rapidly cooled, but the first is not seen with samples cooled slowly [4]. The first and the latter features are also detected by X-ray powder diffraction (when cooling slowly the sample). This last technique showed that the phase transition at 153 K involves a change from room temperature space group C2/m to P2/m.[3] This phase transition seems to be also confirmed by a singularity observed at 150-160 K by IRS with samples rapidly cooled [4]. A 180° flip motion of the W4 water molecule about its quasi-twofold symmetry axis has been found to be responsible for the relaxation in the temperature range 180-335 K by H-NMR. The activation energy of the water flipping motion seems to depend on other factors in addition to the hydrogen-bonding strengths [11]. Previous diffraction studies on single crystals at room temperature [8,10] put into evidence a positional disorder of the water molecules. Two of them are disordered about the symmetry plane (W1 and W2) and another one is located on two different sites, corresponding to general positions within the asymmetric unit, with occupation factors 2/3 and 1/3 (denoted in the following as W3 and W4, respectively). The interpretation of this disorder led to propose the existence of an infinite hydrogen-bonding network, of periodicity three times the cell parameter b, which links two different type of strontium coordination polyhedra along the [0 1 0] direction via the acceptor water molecule W1, the only water molecule not coordinated to strontium ions. Not all sites occupied by hydrogen atoms of W1 could be determined, and this molecule appears to have a free hydrogen atom at room temperature. We have undertaken the neutron diffraction study of SrNP.4H20 at 130 K in an attempt to confirm the space group change from C2/m to P2/m -

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NEUTRON STRUCTURE

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produced by the phase transition at 153 K, detected by X-ray powder diffraction [3,4], and to determine the structural changes produced by this phase transition. This study could contribute to improve the interpretation of results obtained by other techniques. The interpretation of the vibrational behavior of the water molecules at low temperature was made on the basis of crystallographic studies at room temperature but the phase transitions suffered by SrNP.4H20 could bring about concomitant spectral modifications. We report here the average crystal structure in the space group C2/m of SrNP.4H20 at 130 K and its differences with the room temperature structure. EXPERIMENTAL

SrNP.4H20 was obtained in the usual way as described in [4]. Single crystals of dimension appropriated to the study by neutron diffraction were grown by the hanging seed method, by spontaneous concentration of saturated aqueous solutions of SrNP kept in a thermostat slightly above room temperature. Details concerning the crystal data, data collection and refinement conditions are given in Table I. The low temperature was attained cooling the specimen single crystal more o less rapidly within a closed-cycle refrigerator (cooling rate 3 K/min). A contraction of 1.26 % of the a cell parameter is observed while b and e remain constant within experimental error. 250 reflections within the range 3<20<50 ° were measured to confirm the space group P2/m; Fig. 1 reproduces the profiles of selected reflections that are not due to L/2 contamination. As the intensities of the reflections with h+k=2n+l are very weak, data at 130 K were collected in the space group C2/m. The average structure in this space group is certainly a good model for this compound at 130 K. The oxygen atoms of water molecules were located from a difference Fourier map phased with the refined positions and isotropic thermal parameters of all the atoms not belonging to water molecules. Subsequent difference maps gave all positions of the missing hydrogen atoms. All atoms, except hydrogen, were refined anisotropically. The occupation factors of the water molecules were refined to verify the occupation factors 2/3 and 1/3 to W3 and W4 respectively. In the last difference map, the absolute value of the largest residual peak was smaller than 14% of the peak height of a removed carbon atom used as a reference. Structure refinement was performed with the SHELXL93 program [12] on a PC Hewlett Packard Vectra VA.

EUR. J. SOLID STATE INORG. CHEM.

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670

TABLE I. Crystal data, data collection and refinements conditions

Crystal Data Chemical formula F000 Dc (g cm"3) Crystal System Space group a (A)

C~HsFeNrO~Sr 448 2.005 Monoclinic P2/m , refinements w!tl?.C2/rn

b (A)

19.816(24) 7.563(7)

C(A)

8.406(9)

I5(° )

99.02(2) 1244(24) 26 (29 < 20 < 73 °) Orphic reactor, CEN Saclay 1.500(5), ~2 contamination, <010> and<001>

v (A ~) No. of reflections for lattice parameters Radiation Wavelength (~., A) Absorption coefficient (~t cm"1) Temperature (K) Crystal color Crystal size (mm) Crystal description (average zone faces) Data collection Diffractometer Data collection range and scan mode

Index range (omitting h+k = 2n+l) No. of reflections measured Rint No. of independent reflections No. of reflections with I > 2.5~(I) Standard reflections, variation Absorption correction type Transmission factors (Tmin, Tmax) Refinement Refinement method No. of parameters refined Weighting scheme R, R~ for all and observed reflections Goodness of fit, all and observed reflections Extinction correction method Extinction coefficient Sources of atomic scattering factors

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35 -- 1998 -- N°s 10-11

Four-circle diffractometer (6T2 channel) ]o-scan for 3<20<50°, to/0-scan for 50<20<90°, o/20-scan for 90<20<120°, (2-4 s/step, 41 steps) adjusted as a function of the scattering angle to match the instrument resolution (6-8tg0+ 18tg20) -22
NEUTRON STRUCTURE

671

=

"G

# # ~

•

~4~ o

~0

+,

o

[]

o

o

o

o o

0

o

o

o

0

o

o

o

o

o

o o

o 4 -6 -4 peak

2 -3 -2 peek

loo

o -o4

+o.2

o

o2 o4 O m e g a (arbitrary unit)

140o

s O

1

00 iooo

0

0

0

0

0

0

0

D

O

0

0

0

0

~

0

0 0 0

4 •

"* 8 3 - 2 p e a k

-0A

-02

.IP

4. I

•

012 6.4 peak

0

O2

0,4

Omega (arbitrary unit)

Fig. 1. The intensities of the reflections (2 -3 -2) and (6 3 -2), normally unobserved in the space group C2/m, indicate the change to space group P2/m. They are not a L/2 effect. RESULTS A N D DISCUSSION As advanced above, crystal data are included in Table I. Final atomic parameters are presented in Table II. Fig. 2 shows a schema of the hydrogen-bonding network along the [0 1 0] direction and Fig. 3 reproduces the crystal packing. In order to compare the differences between the structures of SrNP.4H20 at 295 K and at 130 K, selected bond distances and angles at both temperatures are presented in Table III. No appreciable changes in Fe and Sr coordination are observed. Superposing the respective coordination polyhedra (at 295 K and 130 K) of Fe and Sr via a least squares process and keeping the Fe and Sr atoms in the same positions, the root mean square difference in the distances between corresponding atoms of the two [Fe(CN)sNO] 2 pseudooctahedra and of the EUR. J, SOLID STATE INORG. CHEM.

672

G. CHEVRtERet al.

two different pairs o f distorted bicapped trigonal prisms around Sr (Srl, Sr2, Fig. 2) are 0.015, 0.034 and 0.028 /~ respectively. These small distortions are due principally to a small displacement o f N2 and 0 4 . The equivalent isotropic thermal parameters are about 36% lower than those found at 295 K. 1 ol

o

to

ol

%

/o

1 o~

~_NI/A~_

&

o

.

Fig. 2. Drawing showing the chains of the two types of Sr polyhedra connected by hydrogen bonds along the [0 1 0] direction. Underlined numbers indicate the different coordination sites for the water molecules Wl, W3 and W4 along 3 x b. TABLE II. Final fractional atomic coordinates, occupation factors and equivalent isotropic thermal parameters with estimated standard deviations in parentheses. Bcq = 4/3 ~]]~fl ~aiaj. Starred atoms were refined isotropically. Atom

Sr Fe O N N(I) N2) N3) C(1) C(2) C(3) O(1) H(11)' H(12)* 0(2) H(21)* H(22)* 0(3) H(31)" H(321)" H(322)' 0(4) H(41)* H(422)* H(421)*

x

y

z

s.o.f

B.q/B~o

.1326(2) .4123(1) .5539(3) .4973(2) .3953(1) .4037(1) .2559(2) .4017(1) .4070(1) .3143(2) .2879(4) .2776(7) .3298(9) .5028(3) .4734(6) .4771(7) .7055(4) .7343(6) .688(1) .739(1) .267(1) .281(1) .225(2) .239(3)

.00000 .00000 .00000 .00000 .2856(3) .2889(4) .00000 .1793(4) .1800(4) .00000 .50000 .50000 .553(2) .50000 .4629(2) .50000 .209(1) .173(1) .097(4) .237(4) .434(3) .428(4) .356(7) .50000

.2953(5) .2207(4) .2642(7) .2450(4) -.0440(3) .4739(4) .1806(5) .0542(4) .3802(4) .1937(5) .1758(9) .070(2) .194(2) .2450(9) .150(1) .324(2) .3549(8) .277(1) .378(3) .461(3) .473(2) .372(4) .479(5) .481(6)

0.5 0.5 0.5 0.5 1.0 1.0 0.5 1.0 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.667 0.667 0.333 0.333 0.333 0.333 0.165 0.165

1.3(2) 1.1(1) 2.5(3) 1.5(2) 2.6(1) 3.2(I) 2.8(2) 1.7(2) 1.8(2) 1.5(2) 3.0(4) 6.0(4) 6.5(5) 1.9(3) 2.6(3) 4.7(3) 2.8(4) 3.8(2) 4.3(7) 2.8(7) 4.4(9) 5.9(9) 1.6(9) 3.3(9)

TOME35 --- 1998 -- N°~ 10-11

673

NEUTRON STRUCTURE

Concerning the water molecules and in comparison with the previous results at room temperature [8] this study at 130 K shows unambiguously a positional redistribution of the hydrogen atoms. Between 295 K and 130 K, the planes determined by W1, W2, W3 and W4 rotate 14.3, 17.2, 5.9 and 26 ° respectively. The two different tetrahedral environments of W1 in the crystal (sites _1 and 2 of the Fig. 2) are completely determined, all hydrogen atoms were l o c a t e d . . H l l lies now on the mirror plane and H12 continues being disordered around the symmetry plane. Wl appears at 130 K as more fixed than at room temperature.

I

I .

~

)X

Fig. 3. SrNP.4H20 crystal packing. The OX, OY and OZ axes correspond to a, 3 x b. and e parameters. Only one position of the disordered H12 and H22 are shown. At room temperature, W2 can be considered as a hydrogen donor having as acceptors N1 and N2V; a trigonal environment is completed by the Sr

EUR. J. SOLID STATE INORG. CHEM.

674

G. CHEVRIERe t

al.

cation. At 130K, as a consequence of the rotation of W2, H22 lies on the symmetry plane, whilst H21 occupies a general position, being disordered around the same plane as H12. Some distances from hydrogen to neighboring atoms are shortened, as it occurs with H21--.NI and H22..-N2 bonds indicating that the contact forces between W2 and those nitrogen atoms are strengthened. H22...N2 v distance is now longer, suggesting a bifurcated hydrogen bond; W2 would have therefore three electron-acceptor neighbors, i.e. N1, N2 and N2 v. This water molecule interconnects therefore the [Fe(CN)sNO] 2 octahedra along the [0 1 0] direction, i.e. the bifurcated hydrogen bond v i a H22 joins two consecutive nitroprusside cations along this direction. TABLE III. Selected bond distances (A,) and angles (°) at 295 K (first column) and at 130 K (second column), without correction for thermal motion according to the riding model. Fe coordination ~

A1

A2 N C(1)

d~9~ 1.65(1) 1.928(9)

d~30 1.664(5) 1.936(4)

Fe

C(2)

1.928(8)

1.925(4)

Fe

C(3)

1 . 9 3 ( 1 ) 1.921(6)

Fe Fe

A1 N C(I) C(2) C(1) C(1) O N(3)

A2 Fe Fe Fe Fe Fe N C(3)

A3 Ang295 C(3) 179.2(6) C(1) ~ 89.2(1) C(2) ~ 89.9(1) C(3) 85.6(4) C(2) ~ 170.8(2) Fe 179.7(5) Fe 179.1(1)

Angl~0 179.7(3) 88.9(2) 90.0(3) 85.5(2) 170.7(2) 178.7(4) 178.8(4)

A1 N C(I)

A2 O N(1)

dz95 1.12(2) 1.148(9)

d l30 1.107(6) 1.145(3)

C(2)

N(2)

1.151(9)

1.149(4)

C(3)

N(3)

1.16(1)

1.144(5)

A1 N N C(I) C(2) N(1) N(2)

A2 Fe Fe Fe Fe C(I) C(2)

A3 Angz95 C(1) 95.0(4) C(2) 94.3(4) C(2) 89.7(3) C(3) 85.1(4) Fe 179.2(7) Fe 179.7(4)

Ang130 94.7(2) 94.6(2) 89.8(2) 85.2(2) 179.8(3) 179.2(3)

Ba coordination ~

A1 Sr

Sr Sr

dz95 2.77(2)

dj3o 2.762(6)

A2 0(2)

d295

dl130

iSr

2.56(1)

2.543(7)

N(2) u 2.70(1) ] 0(3) iv 2.65(1)

2.607(5) 2.639(7)

Sr Sr

N(1)"l 0(4)"

2.66(1) 2.63(1)

2.651(4) 2.614(16)

A2 N(3)

A1

~AI-A3 are the atoms involved. Distances (d, A) and angles (Ang, °) occurring twice due to the existence of a crystallographic symmetry plane are printed in bold type. Symmetry codes (i) -x+0.5,-y+0.5,-z+l (ii) x,-y, z; (iii) x-0.5,y-0.5,z (iv) -x+0.5,-y+0.5,-z. TOME35 -- 1998 -- N°s 10-11

NEUTRON STRUCTURE

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W1, W2, W3 and W4 water molecules Molecule

W1

W2

Atoms involved O1 Hll H121/H12 O1 H11 H121/HI2 02 H21 H22 H22 03 H31 H322 03

W3

H31 H322 03 H31 H321 03 H31 H321 04

W4

H41 H422 04 H41 H421

Site _1

A~

Cb

O-H

H-A

O-A

H31 i H41 N3 ii

_2

0.89(2) 0.88(2) 2.16(1) 2.10(2) 0.97(2) 0.9112)

3.0011) 2.99(1)

0.89(2)0.88(2) 0.97(2) 0.91(2)

2.16(1)2.1012)

3.0011)2.99(1)

0.93( I ) 0.95(1 ) 2.52(1) 2.4611) 0.97(1) 0.90(2) 2.87(1) 2.61(1) 2.5011) 2.6111)

3.42(1) 3.38(1) 3.44(1) 3.3611) 3.4411) 3.3611)

0.92(2) 0.97(1) 2.0011) 1.96(1) 1.13012) 1.05(3) 1.95(2) 1.8013)

2.8611) 2.86(1) 2.93(11) 2.85(1 )

0.92(2) 0.97(1) 1.0012) 1,05(3)

2.00(1) 1.96(1) 1.95(2) 1.8013)

2.86(1) 2.86(1) 2.93(1) 2.85(I)

0.92(2) 0.9711) 2.0011) 1.9611) 0.9411) 0.95(1) 2.38(5) 2.3511)

2.86( 1) 2.86(1) 3.1611) 3.1610)

0.92(2) 11.97(1) 2.00(1) 1.96(1) 0.94~1) 0.95(1) 2.09f2 ) 2.05t2)

2.86(1) 2.86(1) 2.6311) 2.5411)

0.96(2) 0.94(4) 0.95(3) 1.0214)

1.75(2) 1.77(3) 1.89(1) 1.8012)

2.68(1) 2.65(2) 2.7911) 2.90(1)

0.96(2) 0.94(4) 0.98(~2) 0.7~4 /

1.7512) 1.7713) 1.82~2t 1.9613 )

2.68(1) 2.65(2) 2.6311 t 2.54(21

H31 * H31 i~ N3 ~ Sd v N1 N2 N2 ~

3

sr 'v H321 '~ Ol '~i 03 'au

4

SlJv H421 ~i O1 '~ 03 ~

_5

Sr Iv H322 v~i Ol vii 03*

_6

Sr 'v H322 ,'iii Ol '~ 04,~i

7

Sr ~ H321 i

O1 0 4 ~X 8 , SrJX H422 i~ Ol O3 i

"Acceptor atom (as hydrogen-donor) bCoordinated atom (as acceptor of either hydrogen or strontium atom) Symmetry codes (i) x-0.5,y+0.5,z (iv)-x+l,-y,.z+0.5 (vii) x+0.5, y-0.5, z (ii) -x+0.5,-y+0.5,-z (v) x,-y+l, z (viii) -x+ 1.5, -y+0.5, -z+ 1 (iii) x-0.5,-y+0.5,z (vi) x,-y, z (ix) -x+0.5,-y+0.5,.-z

UR. J. SOLID STATE 1NORG. CHEM.

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G. CHEVRIERet al. WI, W2, W3 and W4 water molecules (continued)

O-C

H-H

H-O-H

H-O-C

C-O-C

2.00(1) 1.96(1) 1.46(6)1.41(2) 103(2)103(2) 103(1) 113(1) 70(I) 71(1) 1.75(2) 1.96(1) 145(1) 155(1) 104(2) 100(1) 102(2) 101(2) 2.00(1) 1.96(1) 1.46(6) 1.41(2) 103(2) 103(2) 103(1) 113(1) 84(1) 84(1) 2.00(1) 1.96(1) 123(1) 113(1) 104(1) 100(1)

1~1) 1~1)

2.56(2)2.54(1) 1.56(2)1.48(2) 111(1)106(1) 128(1) 128(1) 121(1) 124(1) 2.65(1)2.64(1) 1.55(2)1.60(3) 109(1)106(2) 119(1) 118(1) 2.38(1) 2.35(1) 85(1) 84(1) 1090) 102(1) 91(1) 103(1) 2.65(1)2.64(1) 1.55(2)1.60(3) 109(1)106(2) 119(1) 118(1) 1.82(2) t.96(1) 90(1) 87(2) 109(I) 104(1) 52(5) 67(1) 2.65(I)2.64(1) 1.45(3)1.47(3) 108(2)100(2) 119(1) 118(1) 1.95(2) 1.80(1) 104(I) 107(1) 127(2) 125(1) 101(2) 103(2) 2.65(1)2.64(1) 1.45(3)1.47(3) 108(2) 100(2) 119(1) 118(1) 1.95(2) 1.80(1) 104(1) 107(2) 127(2) 125(1) 101(2) 103(2) 2.63(1)2.61(2) 1.62(9)1.63(6) 116(7)112(3) 113(2) 112(2) 2.09(2) 2.05(3) 84(2) 90(2) 112(2) 127(3) 105(3) 78(2) 2.63(1)2.61(2) 1.52(7) 1.44(6) 103(5)116(4) 113(2) 112(2) 1.89(1) 2.22(1) 104(1) 106(4) 99(2) 95(1)) 135(3) 132(4)

O-H-A

160(1) 175(I)

160(1) 175(1) 163(3) 162(1) :120(3) 141(1) 163(3) 14l(1)

140(1) 139(!) 154(1) 153(1) 166(1) 178(2) 151(1) 155(2) 154(1) 153(1) 166(1) 178(2) 1130(1)104(1) 154(1) 153(1) 141(2) 143(2) 100(1) 104(1) 154(1) 153(1) 115(2) 109(2) 125(1) 128(1) 165(2) 156(2) 158(4) 123(3) 103(1)92(1) 165(2) 156(2) 137(3) 1.33(5)

To fix the site occupied by H12 and H21, for each position of W1 and W2 in the crystal, it can be assumed that the role of W2 is to reinforce the cohesion of the structure along the [0 1 0] direction. Then, H12 must point towards N1 of a nitroprusside anion and H21 towards the NI of the neighboring nitroprusside anion along the b axes (Fig. 3. Distances H12-.-NI* : 2.82(2)/~, O1...Nl ~ • 3.436(8) A, angle OI-H12-N1 ~ 126(1) °, TOME35 -- 1998 -- N°s 10-11

NEUTRONSTRUCTURE

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symmetry code *: x,-y+l, z). W 4 suffers the most important rotation at 130 K (26°). Consequently, the contacts between Sr polyhedra along [0 1 0], assured by H421...O3 v~ and H422..-O4 ix bonds (Fig.2, sites 8, 4 and 7, 8), are weakened. The small positional changes of W3 have not any consequence on the crystal stability. CONCLUSION The most outstanding consequence of the phase transition suffered by SrNP.4H20 is the diminution of the strength of the hydrogen-bond network binding the Sr polyhedra along of the [0 1 0] direction, due to the rotation o f the W 4 molecule. Contrarily to this effect, the structure cohesion along that direction is reinforced by the binding of consecutive nitroprusside anions via the hydrogen bonds of the W2 molecule. ACKNOWLEDGEMENTS We would like to thank Dr. P.J. A y m o n i n o for critical reading of the manuscript. REFERENCES

1. Irradiation-induced metastable states were first detected for single crystals of Na2[Fe(CN)sNO].2H20, U. HAUSER, V. OESTREICH and H.D. ROHRWECH, Z. Phys. A280, 17 (1977); TH. WOIKE, W. KRASSER, P.S. BECHTHOLD and S. HAUSSIJ-HL, Solid State Comm., 45, 499 (1983); J. Mol. Struc., 114, 57 (1984); Phys. Rev. Lett., 53, 1767 (1984); J. Raman Spectrosc., 17, 83 (1986); M. RODLINGER, J. SCHEFER, G. CHEVRIER, N. FURER, H.U. GUDEL, S. HAUSSUHL, G. HEGER, P. SCHWEISS, T. VOGT, T. WOIKE and H. ZOLLNER, Z. Phys. B, Cond. Matter, 83, 124 (1991). It is now known to occur in single crystals, crystalline powders and solutions of other nitroprussides, H. ZOLLNER, W. KRASSER, TH. WOIKE and S. HAUSSUHL, Chem. Phys. Lett. 161, 497 (1989); J.A. GOIDA, O.E. PIRO and P.J. AYMONINO, Solid State Comm., 57, 175 (1986); SolidState Comm., 66, 1007 (1988); J.A. GUIDA, PJ. AYMONINO, O.E. PIRO and E.E. CASTELLANO, Spectrochimica Acta, 49A, N°4, 535 (1993); and references therein. 2. C.O. DELLA VEDOVA, J.H. LESK, E.L. VARETI'I and P.J. AYMONINO, O.E. PIRO, B.E. RIVERO and E.E. CASTELLANO, J. Mol. Struc., 70, 241 (1981). 3. G. CHEVRIER, A. NAVAZA and J.M. KIAT, Materials Science Forum, 278-281, 648 (1998). 4. G. CHEVRIER, A. NAVAZA, J.A. GUIDA and P. J. AYMONINO, J. Solid State Chem., submitted. 5. E.E. CASTELLANO, O.E. PIRO, A.D. PODJARNY, B.E. RIVERO, P.J. AYMONINO, J.H. LESK and E.L. VARETI'I, Acta Cryst. B34, 2673 (1978). 6. S.R. GONZALEZ, O.E. PIRO, PJ. AYMONINO and E.E. CASTELLANO, Phys.Rev., B36, 3125 (1987). EUR. J. SOLIDSTATEINORG.CHEM.

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