Journal of Neutron Research, March–September 2005 Vol. 13 (1–3), pp. 21–26
Pressure Response on Hydrogen Bonds
in Potassium Hydrogen Carbonate and
Sodium Hydrogen Carbonate
H. KAGIa,*, T. NAGAIb,†, K. KOMATSUa,c, T. OKADAa,b, C. WADAa, J.S. LOVEDAYd
and J.B. PARISEe
a
Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo,
Tokyo 113-0033, Japan; bDepartment of Earth and Space Science, Graduate School of
Science, Osaka University, Osaka 560-0043, Japan; cInstitute of Mineralogy, Petrology, and
Economic Geology, Graduate School of Science, Tohoku University, Miyagi 980-8578, Japan;
d
Department of Physics and Astronomy, The University of Edinburgh, Edinburgh EH93JZ, UK;
e
Departments of Geosciences and Chemistry, SUNY at Stony Brook, Stony Brook, NY
11794-2100, USA
Pressure responses of hydrogen bonding interactions were compared between potassium hydrogen carbonate
(KHCO3) and sodium hydrogen carbonate (NaHCO3). KHCO3 undergoes a pressure-induced structural change
at 2.8 GPa and room temperature whereas no phase transition has been found so far for NaHCO3 up to 11 GPa.
KHCO3 exhibits a substantial decrease in the OZH· · ·O angle with increasing pressure. This change could be a
trigger for the phase transition at 2.8 GPa. By contrast, no significant change in OZH· · ·O angle was observed for
NaHCO3 up to 7 GPa. Based on the observations using neutron diffraction, X-ray diffraction and vibrational
spectroscopy at high pressure, a possible mechanism for the phase transition of KHCO3 is suggested. In addition, the
phase boundary of KHCO3 caused by the pressure-induced transition was determined using in situ Raman
spectroscopy at high pressure.
Keywords: Hydrogen bond; High pressure; Neutron diffraction; Phase transition; Hydrogen carbonate; Earth science
INTRODUCTION
The geometry of hydrogen bond, and particularly the competition between H-bond formation
and H· · ·H repulsion, bears directly on the mechanism of hydrogen incorporation and
the stability of the materials in which the hydrogen is incorporated. Pressure is an excellent
variable for the investigation of the competition between H-bond formation and H· · ·H
repulsion in isochemical systems. The geometry of the H-bond and the stability of
hydrous minerals are of direct relevance to important problems in earth sciences such as
the origin of deep-focus earthquakes, deformation of rocks, etc. We have studied
hydrogen bonding in ionic solid materials at high pressure and the pressure responses of the
structure for layered metal hydroxides and hydrous silicate minerals which can be
*Corresponding author. E-mail: [email protected]
†
Present address: Department of Earth and Planetary Materials Science, Graduate School of Science, Hokkaido
University, Sapporo 060-0810, Japan.
ISSN 1023-8166 print/ISSN 1477-2655 online q 2005 Taylor & Francis Ltd
DOI: 10.1080/10238160412331297782
22
H. KAGI et al.
thermodynamically stable in the deep part of the earth [1 – 4]. Hydrogen bonds in solid
material are strengthened with increasing pressure and at further high pressures a repulsion
between hydrogen atoms can be a trigger of amorphousization [3]. In our continuing
projects on deep earth process, our attention has focused on the pressure-response of
materials with strong hydrogen bonds. This is because some new phenomena such as
symmetrization of hydrogen bond observed for ice at megabar pressure could be expected at
moderately high pressure in materials with strong hydrogen bonding at ambient pressure
conditions [5].
We have chosen two hydrogen carbonate (bicarbonate) compounds having strong
hydrogen bonding with an OZH· · ·O distance of approximately 2.6 Å: potassium hydrogen
carbonate (KHCO3) and sodium hydrogen carbonate (NaHCO3). Despite similarity in their
chemical formulae, the geometrical arrangements of hydrogen bonds are quite different in
these two compounds. KHCO3 consists of a stacking of dimers formed by the fusion of the
two HCO2
3 ions through the formation of hydrogen bonds whereas NaHCO3 has a structure
consisting of infinite chains [6]. These different structural arrangements offer the possibility
of different pressure responses of the hydrogen bonds. KHCO3 undergoes a pressure-induced
structural change at 2.8 GPa and room temperature [7], whereas no phase transition has been
found for NaHCO3 up to 11 GPa. In addition, even at ambient pressure, a phase transition was
reported at 319 K for KHCO3 [8,9].
Recently, we have observed change in the local structure of hydrogen bonds in KHCO3 by
means of neutron diffraction and vibrational spectroscopy at high pressure [10]. In the
present study, we compare the pressure-responses on the hydrogen bond structures between
KHCO3 and NaHCO3 in order to further our insight into the mechanism of the pressureinduced phase transition in KHCO3.
EXPERIMENTAL PROCEDURES
Time-of-flight neutron-diffraction data were collected on the POLARIS beamline at the
UK pulsed-neutron facility, ISIS, at the Rutherford Appleton Laboratory. All measurements
were conducted at room temperature. Deuterated powder samples (KDCO3, NaDCO3)
were loaded into vanadium cans to measure the neutron powder diffraction pattern
at ambient pressure. Data at high pressure were collected using the Paris – Edinburgh
opposed-anvil cell (P –E cell). We used a new cell assembly that enables the use of 4:1
methanol – ethanol mixtures and achieves hydrostatic pressure up to at least 9 GPa [11].
The pressure generated was estimated from the unit cell volume of the samples and
a previously determined equation of state. The data obtained were used for Rietveld
structural refinement with the GSAS program [5,12]. The structural model derived from the
ambient data was then used as a starting point for the refinement of high-pressure structures.
The atomic coordinates and isotropic atomic-displacement parameters of all atoms were
refined.
Raman spectra of KHCO3 at various pressures and temperatures were obtained in order
to determine a phase boundary between low-pressure phase and high-pressure phase of
KHCO3 using a diamond anvil cell. The cell was externally cooled or heated with circulated
water flow. Pressure was measured by the standard ruby fluorescence technique
and temperature was determined from the emf of a W –Re thermocouple attached on the
diamond anvil. A 4:1 methanol/ethanol pressure medium was used for measurements at high
pressure. The incident light was 514.5 nm line of Arþ laser and the power at the sample
surface was approximately 5 mW and individual spectra were obtained with an exposure
of 100 s.
HYDROGEN BOND IN METAL BICARBONATES
23
RESULTS AND DISCUSSION
The unit cell parameters and atomic coordinates obtained by means of Rietveld structural
refinement method allows discussion of the pressure response of bond lengths and bond
angles, particularly those relevant to the hydrogen bonding. As shown in Fig. 1, KDCO3
exhibits a substantial decrease in OZD· · ·O angle with increasing pressure and this change
results in the distortion of hydrogen bonding structure and could be a trigger for the phase
transition at 2.8 GPa [10]. Since the crystal structure of the high-pressure phase of KDCO3
has not yet been solved, the data on the pressure dependence of the OZD· · ·O angle is limited
to 2.8 GPa. By contrast, the present results on the crystal structure of NaDCO3 have shown
that the OZD· · ·O angle of hydrogen bonds in NaDCO3 did not significantly decrease with
increasing pressure up to 7 GPa (Fig. 1). This observation suggests that the local structure
surrounding the hydrogen atoms did not exhibit significant change and that the hydrogen
bonding structure is preserved with increasing pressure for NaDCO3. At present, no pressureinduced phase transition has been observed for NaHCO3 (NaDCO3) up to at least 11.7 GPa
from observations of Raman spectra and X-ray diffraction at high pressure and room
temperature. The absence of pressure-induced phase transition for NaHCO3 is attributed to
the robust structure of hydrogen bonds at high pressure. The differences in pressure response
in NaHCO3 and KHCO3 may be attributed to the difference in the hydrogen bonding
structures: a dimer structure for KHCO3 and polymer structure for NaDCO3.
At ambient pressure, intense Raman bands are observed at 1037, 678 and 637 cm21 due to
the combination of CvO(1) stretch and CZO(3) stretch, the combination of in-plane
bending of O(2)ZCZO(3), and stretching of CZO(2) and in-plane bending of CvO(1),
respectively. By plotting the frequencies of these modes versus pressure, a clear discontinuity
can be observed at the phase transition in the case of KHCO3 [10]. In this study, the phase
relationship between the high-pressure phase and low-pressure phase was determined by
detecting the discontinuity in the pressure-dependence of the Raman frequencies (see Fig. 2).
At constant pressure load, temperature was controlled by flowing hot or cold water through
the metal tubings around the diamond anvil cell. The stability fields of the low-pressure phase
(closed symbol) and high-pressure phase (open symbol) of KHCO3 were determined by
FIGURE 1 Comparison of OZD. . .O bond angles between KDCO3 and NaDCO3. The closed circles represent
KDCO3 [9] and the open circles represent NaHCO3 obtained in the present study.
24
H. KAGI et al.
FIGURE 2 Stability fields of the low-pressure phase (closed symbol) and high-pressure phase (open symbol) of
KHCO3 determined from Raman spectroscopy.
Raman spectroscopy. The phase boundary between the two phases was drawn as a straight
line although some ambiguities exist at the phase boundary presumably as a consequence of
hysteresis at the phase transition. The phase boundary determined here was almost horizontal
in the pressure –temperature dimension suggesting that dP=dT ¼ DH=TDV < 0:
On the basis of these data, we would like to propose a possible structure model for
the pressure-induced phase transition of KDCO3. In general, pressure-induced structural
changes for ionic compounds (AX) results in transformation to an isostructural phase of BX
where B has smaller ionic radius than that of A. Since the ionic radius of K is much larger
than that of Na, it is possible that a high-pressure form of KDCO3 is isostructural with
ambient-pressure phase of NaDCO3. In deed, we can build the NaDCO3-type structure
from the KDCO3 structure by small sliding of structural blocks as shown in Fig. 3. In this
model, we assume that the block slips along the b axis for the distance of 0.5b. The evidence,
a phase boundary between a low pressure and a high-pressure phase of KDCO3 is almost
independent on temperature, may support such a martenstic transformation mechanism.
Furthermore, it is noteworthy that this slip can be accompanied by the change in the hydrogen
bonding structure from the dimer arrangement into the infinite chain arrangement. The next
steps in this project will concentrate on testing the above hypothesis for the high-pressure
form of KDCO3 using both powder and single crystal diffraction data collected at high
pressure.
CONCLUSIONS
X-ray, neutron and vibrational spectroscopic measurements confirmed the occurrence of
reversible pressure-induced phase transition of KHCO3 at 2.8 GPa and room temperature.
In contrast, no pressure-induced phase transition has been found for NaHCO3 up to 11 GPa.
Significant decrease in the OZD· · ·O angle was observed for KDCO3 with increasing
pressure up to the phase transition point, whereas no significant decrease in the OZD· · ·O
angle was observed for NaDCO3. Phase boundary between the high- and low-pressure phases
determined by Raman spectroscopic observations revealed that the temperature dependence
HYDROGEN BOND IN METAL BICARBONATES
25
FIGURE 3 A possible phase transition mechanism of KHCO3. The horizontal direction in this figure corresponds
to the crystallographic a axis and the perpendicular direction corresponds to the b axis. (a) Low-pressure phase of
KHCO3. The block slips to the direction denoted by the arrow at the transition point. (b) A possible model for the
high-pressure phase of KHCO3. The hydrogen bonds form infinite chains.
of transition pressure is almost constant with increasing temperature within the observed
temperature range ð270 – 350 KÞ:
Acknowledgements
This work is supported in part by a Grant-in-Aid for Scientific Research (13554018,
14654096, 15340190) from Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan/U.K. Cooperation on the neutron scattering research
program and the US-NSF. We are grateful to Dr R.J. Nelmes for full access to the Paris–
Edinburgh facilities and techniques at ISIS, and to the Rutherford Appleton Laboratory for
the provision of ISIS beamtime. We are grateful to Dr G.P. Glasby for reviewing the
manuscript.
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H. KAGI et al.
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