Optical absorption and tle yield stress value of NaCI : Ni 2+ crystals were measured as functions of tle annealing temperature. Morphology and possible structure types of the ΝiCl2 particles formed under different annealing conditions have been shortly discussed. PACS numbers: 61.72.-y, 78.20.-e, 78.50.-w     Introductory information It was evidenced previously  (363

D Νοwaκ-Wοźνυ

M W Suszyńsκα

T Morawska-Kowal

Trzebiatowski

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Vol. 86 (1994)	 ACTΛ PHYSICA POLONICA A 	 No. 3
PRECIPITATION OF NICKEL DIHALIDE
IN THERMALLY PRETREATED
NaCI : Ni 2+ CRYSTALS
T. MORAWSKA-KOWAL, D. ΝΟWAΚ-WΟŹΝΥ AND M. SUSZYŃSΚΑ 	 .
W. Trzebiatowski Institute of Low Temperature and Structure Research
Polish Academy of Sciences, Okólna 2, 50-950 Wroclaw, Poland
(Received May I7, I994; revised version July 5, 1990
Optical absorption and tle yield stress value of NaCI : Ni 2+ crystals were
measured as functions of tle annealing temperature. Morphology and pos-
sible structure types of the ΝiCl2 particles formed under different annealing
conditions have been shortly discussed.
PACS numbers: 61.72.—y, 78.20.-e, 78.50.-w
1. Introductory information
It was evidenced previously [1-5] that the absorption spectra of nickel-doped
NaCl crystals are sensitive to both the concentration and the dispersion form
of the dopant. In order to obtain more details about the aggregation- and/or
precipitation-related phenomena Bridgman-grown NaCI : Νi 2 + crystals were solu-
tion-treated, i.e. air-quenched after 30 minutes of annealing at 873 K, and addi-
tionally annealed at temperatures ranging between room temperature (RT) and
the solution treatment temperature (ST). For aim of comparison, as-received (AR)
samples were also tested. The dopant concentration, equal to 160 ppm, was deter-
mined from the absorption coefficient of the main absorption band (αC) according
to the method described elsewhere [6]; the ppm is expressed as the number of
ΝiCl2 moles per 10 6 moles of NaCI. For these samples the yield stress value was
critically compared with some data obtained from the optical absorption spectra.
The stress/strain curves were taken at RT by using the 1112 INSTRON machine
working at the strain rate of 3 x 10'4 s -I ; the y ield stress value (σο) was deter-
mined at 0.1% of plastic strain. Optical absorption spectra were measured in the
spectral range from 50 x 10 3 to 11 x l0 3 cm -1 by employing the SPECORD M-40
(Zeiss) spectrophotometer.
(363)
364	 Τ. Μorawska-Kowal, D. Nowak-Woźny, M. Suszyńska
2. Results and discussion
2.1. Optical absorption
Spectral characteristics of the detected absorption bands have been collected
in Table I for as-received, solution-treated and samples additionally annealed (AA)
after the solution treatment. The oscillator strength vahes fi were calculated ac-
cording to the Smakula equation for Gaussian bands with the refractive index-value
taken for RT [7].
Precipitatiοn of Nickel Dihalide ...365
Figure 1 presents the absorption spectra of ST-samples isochronally annealed
(50 h) at temperatures between 323 and 623 K. Especially interesting are the re-
sults obtained for ST-samples additionally annealed at temperatures above 473 K.
While the AR- and ST-spectra consist of three (A, B, C) and two (A, C) strong
bands, respectively, the AA-spectra exhibit the presence of an additional band
(D) located at about 276 nm. The absorption coefficient of this band (αD) in-
creases during the annealing and this increase is accompanied by the decrease in
αC characteristic of band C. It seems reasonable to assume that the centres re-
lated to D-type optical transitions appear at the expenses of centres related to the
characteristic C-type absorption.
2.2. θ.1% proof stress
Figure 2 shows the effect of annealing time at different annealing tempera-
tures upon the yield stress value of AA-crystals (σ0(t)) with respect to the ST-ones
(σ0(SΤ)); the annealing temperature was chosen from the range between RT and
673 K. It has been stated that the value of Δσ0 = σ0 (t) - σ0 (ST) is positive for
AA-samples annealed at temperatures between  RT and 473 K; for higher annealing
temperatures Δσ0 = 0.
The optical (αC, αD) and mechanical (Δσ0) characteristics of isochronally
annealed samples have been compared in Fig. 3. Although the yield stress value of
samples annealed at high temperatures (at and above 473 K) is equal to the value
characteristic of ST specimens (Δσ0 = 0), the absorption spectra of both kinds of
crystals differ from each other. Thus, the relative softening of AA-samples cannot
be related to thermal decomposition of the obstacles effective after annealing at low
temperatures; these obstacles are related to NiCl 2 particles formed in NaCl crystals
at about 323 K [2]. The effect of the high temperature annealing is explainable
in terms of a NiCl 2 NiC1*2transformation, where the ΝiC1p rticles could be
considered as the "vacancy-rich" NiCl2 phase suggested by Andreev at al. [8].
366 	 Τ. Morawska-Kowal, D. Nowak- Woźny, Μ. Suszyńska
2.3. Precipitalion-related phenomena
Some aspects of the precipitation of nickel dihalide phases were investigated
by using an Avrami-type equation in which the fraction of the dopant transformed
into precipitated particles (p) was determined in the way previously proposed for
NaCl : Eu 2 + crystals [9].
Figures 4 and 5 show the isothermal n-values (n = 1 - p) determined from
the optical and mechanical data, respectively, and Table ΙI collects valnes of the
Avrami parameter m [10]. According to the precipitation models [11, 12], the pre-
cipitation rate below 403 K, where m < 1, should be driven by a dislocation-aided
Precipitalion of Nickel Dihalide ...	 367
process, whereas above this temperature bulk diffusion and processes at the par-
ticle/matrix interface should be dominant (m = 1.5 ÷ 2.0). This means that the
strengthening effect is characteristic of this annealing temperature range for which
m < 1, whereas m > 1 correlates with Δσo = 0.
2.4. Structure of the ΝiCl*2 phase
In order to explain the correlations between precipitation and strengthening
phenomena let us assume that both nickel dihalide particles formed under different
annealing conditions are of the same geometrical shape. In terms of the considered
precipitation models [11, 12], they should be rods oriented along (110) direction.
Ιn fact, this shape was confirmed experimentally by some microscopic observations
[13, 14].
If so, the high temperature phase should have the same crystallographic
stucture as the low temperature one, i.e. the hexagonal unit cell of the CdCl2-type.
368	 Τ. Morawska-Kowal, D. Nowak- Woźny, Μ. Suszyńska
Because of some differences in the density of NaCl crystals containing either NiCl2
or ΝiCl *2,considered as equivalent to tle "vacancy-rich" NiCl2 phase [8], one has
to expect that the lattice parameter  h of tle high temperature phase is slightly
larger than h characteristic of tle low temperature NiCl 2 .
Ιn order to estimate the  h*-vaIue, the elementary cell of NiCl 2 was matched
with that of NaCl. it was assumed tlat the sixfold rotation axis of the NiCl2 ele-
mentary cell is parallel to tle shortest slip vector of tle matrix, d( 110). Α matcling
parameter was estimated on the basis of the following relation:
ε = (n* — n)/d( 110 ),
where n* = hnhex/d(11O), n is tle entire part of n* and n hex is tle number of
hexagonal NiCl2 cells along the (110) direction. This parameter described the
misfit parameter for a  group of n h eΧ NiCl 2 elementary cells.
The smallest value of ε (0.04) was obtained for a group of 7 NiCl2 cells. On
assuming that tle ε parameter for 7 cells of the ligl temperature phase is equal
to zero, tle new parameter  of h* equals 17.38 Α.
3. Summary
In additionally annealed solution-treated NaCl : Ni 2 + crystals the dopant is
precipitated in two forms of the nickel dihalide. Both phases are hexagonal with a
slightly different lattice parameter  "ħ" . Τhe transformation occurs at temperatures
equal and higher than 473 Κ and involves tle decrease in crystal density.
This work has been done in frames of the project Νo. 2 Ρ302 17δ 0Λ of the
Committee for Scientific Research.
References
[1]K. Polak, Ζ. Phys. 223, 338 (1969).
[2] D. Νowak-Woźny, Μ. Suszyńska, Ada Phys. Pol. Α 81, 419 (1992).
[3] Μ. Suszyńska, Science Forum 126-128, Λ19 (1993).
[4] Μ. Suszyńska, R. Capelletti, Mater. Chem. Phys. 34, 228 (1993).
[5] D. Νowak-Woźny, Μ. Suszyńska, Μ. Szmida, R. Capelletti, J. Mater. Sci. 28, 645
(1993).
[6] D. Νowak-Woźny, PhD Tlesis, Inst. of Low Temperature and Structure Research,
Polish Academy of Sciences, Wroclaw 1994.
[7] The Physicochemical Handbook, Eds. J. Gajewska, S. Pietras, J. Rudzińska, Α.
Schellenberg, PWN, Warszawa 197Λ (in Polish).
[8] Α. Αndreev, M. Hartmanoνa, V.A. Klimoν, Phys. Status Solidi Α 41, 697 (1977).
[9] D. Νowak-Woźny, Cryst. Res. Technol. 24, 979 (1989).
[10] Μ. Avrami, J. Chem. Phys. 9, 177 (194!).
[11] C. Zener, Trans. A.I.M.E. 167, 550 (19.16).
[12] C. Wert, J. Appl. Phys. 20, 943 (1949).
[13] Α. Andreev, M. Hartmanova, V.Λ. Klimov, Phys. Status Solidi Α 92, 35 (1980).
[14] M. Szmida, private com m unications.


PRECIPITATION OF NICKEL DIHALIDE IN THERMALLY PRETREATED NaCI : Ni2+ CRYSTALS

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PRECIPITATION OF NICKEL DIHALIDE IN THERMALLY PRETREATED NaCI : Ni2+ CRYSTALS

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