Powder neutron-diffraction profile analysis of zero-dimensional H-bonded crystal HCrO2

doi:10.1016/S0022-3697(99)00189-4

Journal of Physics and Chemistry of Solids

Volume 60, Issue 11, November 1999, Pages 1875-1880

https://doi.org/10.1016/S0022-3697(99)00189-4 Get rights and content

Abstract

Powder neutron-diffraction profile analysis of α-HCrO₂ has been done at five temperatures, 295, 210, 140, 70 and 10 K, to confirm the space group and the hydrogen-bond distance. The data were analyzed for three models: (1) space group $R 3 ̄$ m with off-centered disordered hydrogen; (2) $R 3 ̄$ m with centered hydrogen; and (3) R3m. The space group $R 3 ̄$ m with off-centered disordered hydrogen gave the most reasonable results in the range of 70–295 K, consistent with earlier results. The hydrogen bond distance 2.471(3) Å at 295 K agrees with one of the earlier studies. The space group at 10 K is less conclusive.

Introduction

α-HCrO₂ and α-HCoO₂ are isomorphous, with trigonal symmetry and NaHF₂ structure (Fig. 1). These are extremely simple systems with only one HMO₂ and hydrogen bond in the asymmetric unit [1]. This greatly simplifies the interpretation of spectroscopic data. HCrO₂ has attracted much interest spectroscopically because of the anomalous deuteration effect on the vibrational spectra. An infrared study of HCrO₂ and DCrO₂ [2] revealed some unexpected effects: (1) whereas the O–H stretching band occurs at 1640 cm⁻¹, the O–D stretching band appears at 1750 cm⁻¹ (ν(O–H)/ν(O–D)=0.94); (2) the O–H stretching band occurs as a singlet, but the O–D band as a doublet; (3) the frequency ratio of the OHO bending mode to the ODO bending mode at room temperature is 1.441, larger than other hydrogen-bonded systems; and (4) there are two bands in the lattice mode region in HCrO₂, but six bands in DCrO₂. These differences were ascribed to a difference in the potential energy curve between O–H–O and O–D–O bonds. On the basis of the potential with Ax²+Bx⁴ type, Snyder and Ibers [2] concluded that the potential energy curve of the O–H–O bond is of an effectively single-minimum type and that of the O–D–O bond is of double-minimum type. The infrared spectra were also analyzed using a model potential consisting of two back-to-back Morse function and the well parameters were determined [3], [4]. Extensive infrared and inelastic neutron scattering measurements of these systems have been made [5], [6], [7], [8], [9], [10]. The system HCoO₂–DCoO₂ behaves in a completely analogous manner [11].

Hamilton and Ibers [1], [12] analyzed the structure of HCrO₂ and DCrO₂ by neutron powder diffraction and the results were compatible with the interpretation of the infrared spectra. They drew the conclusion that (1) both HCrO₂ and DCrO₂ have the common space group of $R 3 ̄$ m and that the deuterium atoms are off-centered and disordered in DCrO₂, but no conclusion could be reached concerning centering or off-centering of hydrogen atoms in HCrO₂, (2) the O–D–O bond distance (2.55(2) Å) is 0.06 Å longer than the O–H–O bond distance (2.49(2) Å). More recently Christensen et al. [13] did neutron powder profile refinement of both HCrO₂ and DCrO₂ and concluded: (1) that the space group of HCrO₂ is $R 3 ̄$ m, consistent with Hamilton and Ibers, and with the hydrogen atoms off-centered and disordered, but that the space group of DCrO₂ is R3m and deuterium atoms are off-centered and ordered; (2) that the expansion of the O⋯O distance is 0.10 Å with an O–H–O bond distance of 2.47(1) Å and an O–D–O bond distance of 2.57(2) Å.

As described in the textbook of Hamilton and Ibers [1], the questions of interest in the HCrO₂ and DCrO₂ systems are

1.
The type of potential curve: Is O–H–O (O–D–O) bond of single-minimum or double-minimum type?
2.
An extraordinarily large expansion of the hydrogen bond on deuteration (geometric isotope effect [14]): Is this true and, if so, how can it be explained?

If the reported isotope effect is really true, the question is: why are HCrO₂ and HCoO₂ so exceptional? We notice some characteristic structural features in these systems. Firstly, the hydrogen-bond unit OHO along the c-axis is located in a layer perpendicular to the c-axis and sandwiched by a Cr atom layer. In H-bonded crystals the observed expansion of the O⋯O distance is in general a result of a compromise with other effects in the environment surrounding the OHO system (like cation coordination polyhedra). The geometric isotope effect is thus smeared out or suppressed due to such a situation in the crystals. The separated layer structure of the HCrO₂ system seems to allow the hydrogen bond to expand without competing with other factors. Secondly, considering the hydrogen-bond network, the HCrO₂ system is one of the simplest compounds with 0-dimensional hydrogen-bond network (isolated H-bond system) [15], [16], [17]. Thus we may say that HCrO₂ is a “ultimate” 0-D system.

The magnitude of the geometric isotope effect in the HCrO₂ system is, therefore, in the range of 0.06(3)–0.10(2) Å, depending on the combination of the space group and the literature data adopted [12], [13]. This would mean that the geometric isotope effect is 2–3 times larger than the largest geometric isotope effect observed in other H-bonded systems [14], [18]. However, judging from the combined standard deviations, it seems difficult to draw a definite conclusion. In view of these interesting questions, we have started a systematic study on the HCrO₂ and DCrO₂ systems. In this article the results of neutron powder-diffraction of HCrO₂ is reported.

Section snippets

Experimental

Green-grey powder of HCrO₂ was prepared from reagent material (Cica) of Cr(OH)₃·nH₂O by adding 0.5 N NaOH and application of hydrothermal synthesis technique (360°C, 44 MPa, 10 days). The synthesized powder was dried at 100°C after washing with distilled water. Diffraction data were taken on the neutron powder diffractometer at the Studsvik Neutron Research Laboratory, Sweden. The diffractometer is equipped with a bank of 35 detectors and an Air Products closed-cycle helium refrigerator system. A

Results and discussion

All the data were analyzed for three different models, namely (1) space group $R 3 ̄$ m with off-centered, disordered hydrogen atoms, (2) space group $R 3 ̄$ m with centered hydrogens and (3) space group R3m with off-centered, ordered hydrogens. Comparison between the three models are given for the 295 and 10 K data in Table 1. Observed, calculated and difference diffraction patterns of HCrO₂ at 295 and 10 K are shown in Fig. 2 for the $R 3 ̄$ m model with off-centered, disordered hydrogen atoms. As far as the

Acknowledgements

We wish to thank Håkan Rundlöf for skilful assistance in the data collection of the neutron diffraction data.

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Powder neutron-diffraction profile analysis of zero-dimensional H-bonded crystal HCrO2

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Acknowledgements

Spectrochim. Acta

J. Solid State Chem.

J. Mol. Struct.

J. Mol. Struct.

Physica B

Chem. Phys. Lett.

Hydrogen bonding in solids

J. Chem. Phys.

Ferroelectrics

Mol. Phys.

J. Chem. Phys.

Powder neutron-diffraction profile analysis of zero-dimensional H-bonded crystal HCrO₂