Neutron powder diffraction was applied to determine the deuteration level of K(H1−xDx)2PO4 (DKDP) crystals via the Rietveld refinement method. Micro-Raman spectroscopy was employed using the neutron results to investigate the relation between deuteration level and PO4 vibration peak of DKDP crystal. The relative variation [Δν1 = ν1(KDP)−ν1(DKDP)] of the PO4 vibration peak was linearly well dependent on the deuteration level. Attenuated Total Reflectance-Infrared (ATR-IR) spectroscopy was also used to study the total relative variation [β(DKDP)-β(KDP) + ν1(DKDP)−ν1(KDP)] of β(O-H/D) and ν1(PO4) absorption band with the deuteration level of DKDP crystals. The IR results illustrate that two linear relationships existed between the deuteration level and the relative variations of the spectra. The two spectroscopic techniques can be combined and used to measure the degree of deuteration in the crystals of DKDP grown from solutions with deuteration level of less than 92%. ATR-IR spectroscopy is more suitable for measuring highly deuterated DKDP crystals.
© 2016 Optical Society of America
Potassium dihydrogen phosphate (KDP) with excellent frequency conversion and electro-optic effect is used in inertial laser fusion facilities [1–4]. Its isomorph, potassium dideuterium phosphate (DKDP), is widely used to suppress transverse stimulated Raman scattering [5,6]. Different degrees of substitution can alter the structural dimensions, bond lengths, and bond angles with the deuterium composition in crystals, although the difference between hydrogen and deuterium atoms is very little [7–9]. These chemical and physical properties are closely related with the deuteration level of DKDP crystals. Therefore, an accurate measurement of deuteration level is very important because of its applications.
Loiacono et al.  proposed the variation in ferroelectric transition temperature of DKDP crystal to determine its deuteration level. They reported that the ferroelectric transition temperature of the DKDP crystal was almost dependent linearly on its deuteration level. However, most experimental data are concentrated on the high deuteration level, whereas only few test data on mid-low deuteration range are available. Thus, this proposed method is much more accurate to measure the deuteration level of highly deuterated DKDP crystals. Yaksin et al.  suggested Raman shift to measure the deuteration level. Huser et al.  found that the relationship between the deuteration level and Raman shift of PO4 vibration peak is almost linear. Thus, micro-Raman spectroscopy is inferred to be suitable for measuring the deuteration level. However, this study would be more accurate if the relative variation would be used in the Raman shift of PO4vibration peak . Thermal gravity analysis (TGA) was used in 2006 to measure the deuteration level depending on the thermo-decomposition of DKDP and KDP crystals .
Neutron diffraction is an effective technique to determine the deuteration level of DKDP crystals because of the sensitivity of the neutron to hydrogen and deuterium elements. Neutron diffraction was applied in 1988 to analyze the structural difference between DKDP and KDP crystals and determine the deuteration level in highly deuterated DKDP crystals . Neutron diffraction is rarely used in studies, although this process is very useful in determining the deuteration level. Therefore, neutron diffraction can be used as a standard method to calibrate the deuteration level of DKDP crystals.
In the present work, the deuteration level of DKDP crystals was determined systematically by neutron diffraction. Micro-Raman and ATR-IR spectroscopies were applied using neutron diffraction results to investigate the dependence of deuteration level on the variation in Raman and IR spectra, respectively. These two spectral techniques can be used conveniently to determine the deuteration level of DKDP crystals. Both techniques can be used to measure the deuteration level of DKDP crystals grown from solutions with deuterium content of less than 92%. However, ATR-IR spectroscopy is more appropriate to measure more highly deuterated DKDP crystals.
2. Experimental section
2.1 Crystal growth and sample preparation
The DKDP crystals were grown from solutions with deuteration levels of 0%, 40%, 55%, 65%, 80%, 92%, 98%, and 99.5%. The deuteration level in the growth solution (Ds) is calculated as follows:16] and . The growth of all DKDP crystals was described in detail in Refs  and .
2.2 Neutron powder diffraction
Neutron powder diffraction experiments were performed using the high-pressure neutron powder diffractometer (HPNPD) at the Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics. The diffraction data were collected in the 2θ scan mode from 10° to 150° by the HPNPD with a monochromatic neutron beam with wavelength of 1.57639 Å generated from a Ge(115) monochromator. The neutron diffraction patterns obtained at room temperature were refined by Rietveld method using the FullProf.2k program .
2.3 Micro-Raman spectroscopic measurements
Raman spectral studies were conducted at room temperature using laser micro-Raman technique (LabRAM HR evolution) with an excitation wavelength of 532 nm. The spectral resolution in the experiments was approximately 0.7 cm−1. All Raman spectra were obtained by focusing the laser beam as deep as 40 μm into the bulk of the crystals to avoid contributions from the potentially influential layers by hydrogen in air. A single Si crystal was applied as a standard to calibrate the spectrograph before the spectra were obtained. The spectra were analyzed by fitting the PO4 Raman peak with a single lorentzian to determine the peak position. The error of these fitted peak positions is about 0.07 cm−1.
2.4 ATR-IR spectroscopic measurement
The spectral studies of powder specimens with size of approximately 1 μm were performed at room temperature using ATR-IR technique (Thermo Nexus5700) with an ATR cell. The internal reflection element (IRE) was a diamond crystal. FT-IR spectrometer was used to measure the absorption of all DKDP powder specimens in the spectral range of 4000–600 cm−1. The spectral resolution used in the experiments was approximately 1 cm−1. The spectra of all DKDP samples were displayed in the form of transmittance spectra.
3. Results and discussion
3.1 Determination of deuteration level of crystals via neutron diffraction
The deuteration level of grown DKDP crystals always differs from their mother solutions. Hydrogen and its isotope deuterium can be readily distinguished by neutrons. Hence, the degree of deuteration in crystals (Dc) can be actually obtained from the refined neutron diffraction data (Fig. 1). The Rietveld structural refinement method is used to refine the occupational ratio of hydrogen and deuterium to minimize the R factors. Table 1 shows the deuteration levels of the crystals grown from various deuterated level solutions and the refined R factors. The relatively small R-factors can well demonstrate the high quality of agreement between the observed and calculated profiles, which indicates that the determination of deuteration level is accurate.
Figure 1 shows that the intensity of the (200) reflection decreases with increasing deuteration level of the DKDP crystals. This result indicates that the substitution of D for H increases the separation between (200) lattice planes. Hydrogen or deuterium bond is vertical to (200) face as shown by the cell structure in Fig. 2. Table 1 shows that the (O-O) distance (Fig. 2(b)) is affected largely by the incorporation of deuterium in the crystals, the value of which increases with increasing deuteration level of the DKDP crystals. The change would affect the bond of hydrogen or deuterium and oxygen. Meanwhile, the bond length of (P-O) varies with the deuteration level of the DKDP crystals. These variations can cause spectroscopic change caused by the substitution of hydrogen by deuterium.
3.2 Raman spectrum
Neutron diffraction is very useful in determining the deuteration level in crystals, but this process is rarely performed. Thus, neutron diffraction should be used as a standard to determine the degree of deuteration of crystals. The position of the asymmetric P(OD)2 stretching vibration ν1 exhibits a linear dependency to H/D substitution. Hence, micro-Raman spectroscopy is frequently applied to conveniently determine the deuteration level of crystals according to the change in PO4 vibration peak . The accuracy of the quantitative correlation between the degree of deuteration and Raman shift of the PO4 vibration peak is very critical to determine the deuteration level of crystals.
The observed main features are the Raman shifts of the PO4 vibration peak of the KDP crystal at 914.9 cm−1 and the shift toward shorter wavenumber of 878.8 cm−1 as deuteration levels of crystals increase to 99.1% (Fig. 3(a)). And, the intensity of the peak at about 960 cm−1, which presents in-plane bending mode of the hydrogen or deuterium bond , increases with increasing the deuteration level in DKDP crystals. Due to a Fermi resonance of a weak O-D bending mode , in KDP corresponding O-H bending is very weak, and its resonance with the 960 cm−1 line is practically zero, while in DKDP corresponding O-D bending become stronger with higher deuteration level. Investigating the dependence of relative Raman shift [Δν1 = ν1(KDP)−ν1(DKDP)] instead of the absolute Raman shift of crystals can be more useful to determine the deuteration level in this method to minimize errors from micro-Raman spectroscopy. Figure 3(b) shows that the dependence of the relative Raman shift for DKDP on the degree of deuteration Dc is almost perfectly linear, as follows:
The following three steps are performed using the results to measure the deuteration level via micro-Raman spectroscopy. First, the Raman equipment was calibrated using a single crystal Si. The Raman spectra of the KDP and DKDP crystals were then measured. Finally, Eq. (2) was used to calculate the deuteration level of the DKDP crystals.
Based on our experimental data the formula describing the dependentce of the relative Raman line width (W) of the PO4 vibration peak on deuteraion level can be fitted shown in Fig. 4 using lorentzian function:
3.3 IR spectrum
The neutron diffraction results show that the bond lengths of P-O and O-H/D varied with the different deuteration levels in the DKDP crystals. This change can make the β(O-H/D) (it presents stretching vibration of the O-H or O-D bond) and ν1(PO4) of their IR spectra shift toward larger wavenumber as the deuteration level of the crystals increased (Fig. 5(a)). The observed absorption wavenumber of β(O-H) of KDP was 1273.8 cm−1, and it shifted toward a larger wavenumber of 1300.5 cm−1 when deuteration level increased to 99.1% to become β(O-D). Similarly, the trend of ν1(PO4) was the same as that of β(O-H/D). The absorption wavenumber of ν1(PO4) is 828.7 cm−1 for KDP, whereas it become to 915.8 cm−1 for the 99.1% deuterated DKDP crystal. Therefore, we can measure the deuteration level of DKDP crystals depending on the relationship between the deuteration level and the variation in β(O-H/D) and ν1(PO4) of the IR spectra of DKDP crystals.
Table 2 and Fig. 5(b) show that two kinds of linear relationship between the relative total change of β(O-H/D) and ν1(PO4) and deuteration level. When deuteration level is less than 73.8%, degree of deuteration Dc is almost perfectly linear with the toatal wavenumber variations of β(O-H/D) and ν1(PO4), as shown in the following Eq.:
The following two steps can be used to measure the deuteration level of DKDP crystals via IR spectroscopy. First, the IR spectra of the KDP and DKDP crystals were measured. Then, Eq. (4) and Eq. (5) were used to calculate the deuteration levels of the DKDP crystals.
3.4 Comparison the accuracies of micro-Raman and ATR-IR spectroscopy
Deuteration level can be determined by micro-Raman and ATR-IR spectroscopies depending on Eq. (2), Eq. (4) and Eq. (5). Table 3 shows that the deuteration level determined by micro-Raman spectroscopy and Eq. (2) is less than that measured by neutron diffraction except for Ds = 92%. The mix-difference appeared at lowly and highly deuterated level of DKDP crystals, which is approximately 4%. However, the deuteration level measured by ATR-IR spectroscopy based on Eq. (4) and Eq. (5) is more than that that measured by neutron diffraction except for Ds = 92%. The max-difference exists at low deuteration level. Consequently, both micro-Raman and ATR-IR spectroscopy can be combined and used to determine the deuteration level in DKDP crystals for measuring Ds≤ 92%. ATR-IR spectroscopy is better than micro-Raman spectroscopy for higher deuteration levels.
Neutron powder diffraction was applied to measure the deuteration of DKDP crystals. The deuteration level was obtained using the Rietveld method to refine the neutron diffraction data patterns. Micro-Raman spectroscopy of the bulk sample was applied at room temperature using the neutron results to investigate the relationship between the deuteration level and the relative wavenumber variation in the PO4vibration peak shift [Δν1 = ν1(KDP)−ν1(DKDP)]. One wavenumber variation in the Raman shift could be attributed to 2.64% deuteration. ATR-IR spectroscopy of the powder sample at room temperature was studied, and the results show two relationships between the deuteration level and the total relative variation in β(O-H/D) and ν1(PO4) absorption bands. This result indicates that a change of one wavenumber was caused by 0.748% deuteration when the deterioration level was less than 73.8%. By contrast, 1.681% deuteration could cause one wavenumber variation at deuteration level of more than 73.8%. Both micro-Raman and IR spectroscopy could be used to determine the deuteration level of DKDP crystals for crystals grown from solutions with deuteration level of less than 92%. However, IR spectroscopy presents more accurate results for highly deuterated DKDP crystal.
This work is supported by the National Natural Science Foundation of China (Grants No. 51323002 and 51402173), the Ministry of Education (Grants No.625010360), the Project 2014BB07 from NPL, CAEP in China and the Fundamental Research Funds of Shandong University (Grants No.2015GN027).
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