Anomalous second-harmonic generation response in SrBPO5 and BaBPO5

Article abstract of DOI:10.1039/c4tc02443e

In order to study the mechanism of the second harmonic generation (SHG) effect in borophosphates, two large ultraviolet nonlinear optical crystals, SrBPO₅ and BaBPO₅, have been successfully obtained using a top-seeded solution growth method. Optical and thermal properties have been measured, and SrBPO₅ and BaBPO₅ are shown to display large experimental gaps (6.44 eV and 6.14 eV, respectively) and moderate SHG coefficients (0.57 and 0.43 times that of KH₂PO₄). More importantly, based on a further understanding of the relationship between nonlinear optical properties and crystal structure within density functional theory (DFT), we have demonstrated that BO₄ groups and metal cations play an essential role in the SHG effect of SrBPO₅ and BaBPO₅, but not PO₄ groups as anticipated. Indeed, PO₄ groups play a counteractive role in BaBPO₅ due to the indirect influence of metal cations.

Full text of DOI:10.1039/c4tc02443e

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ARTICLE
Anomalous Second Harmonic Generation (SHG)
Response in MBPO (M=Sr, Ba)
5
Cite this: DOI: 10.1039/x0xx00000x
a,b
a,b
,a
a,b
,a
BingꢀHua Lei, Qun Jing, Zhihua Yang,* Bingbing Zhang, Shilie Pan*
5 5
Large sized ultraviolet (UV) nonlinear optical (NLO) crystals SrBPO and BaBPO have been
Received 00th January 2012,
Accepted 00th January 2012
obtained successfully by top-seeded solution growth method in order to study the mechanism of
second harmonic generation (SHG) effect in borophosphates. Optical and thermal properties
DOI: 10.1039/x0xx00000x
have been measured. SrBPO
and moderate SHG coefficients (0.57 and 0.43 times of KH
on the further insight into the relationship between nonlinear optical properties and crystal
structure within density functional theory (DFT), we discover that BO groups and metal cations
take essential roles in SHG effect of MBPO (M=Sr, Ba) but not PO groups as expected.
groups play a counteractive role in BaBPO because of indirect influence from
5
and BaBPO
5
display large experimental gaps (6.44 and 6.14 eV)
www.rsc.org/
2
PO (KDP)). More importantly, based
4
4
5
4
Moreover, PO
metal cations.
4
5
Introduction
exhibit rich chemistry structure and many potential
1
5ꢀ18
applications
. The crystal chemistry of borophosphates with
The solid state laser has many significant applications in a
broad area, such as laser micromaching, semiconductor
photolithography,
anionic partial structures mainly built from tetrahedral building
units reveals a large structural variety, similar to that of silicates
and alum silicates. The primary BO , BO and PO building
photochemical
synthesis,
laser
4
3
4
communication and so on. Owing to the ability to shorten the
wavelength of light based on the process of secondꢀharmonic
generation (SHG), the nonlinear optical (NLO) crystals become
the key material used in the frequency conversion device. After
continuous efforts over several decades, many excellent NLO
crystals were obtained. The most advanced, commercially
available benchmark NLO materials for ultraviolet−visible
units can interconnect to form complex anionic structures from
isolated species, oligomers, rings, chains, layers to
framework . Therefore, there is lots of room to explore novel
19
compounds for the considerable variety in the chemical
structure. For example, the MBPO (M=Sr, Ba, Ca) compounds
5
belong to the stillwellite structural type, and contain single
chains of the BO tetrahedra running parallel to [001] which are
4
(
(
UVꢀvis) region are borates or phosphates, such as KH PO
20, 21
2
4
4
linked to terminal PO tetrahedra to form spiral chains
BPO whose structure is similar to that of silica also contains
BO and PO groups. Except for tetrahedrally coordinated B
and P , Co BP O is the first borophosphate with planar BO
3
.
4
1
2,
3
KDP)
,
KTiOPO4 (KTP)
5
,
βꢀBaB O4 (BBO) ,
22
2
4
LiB O (LBO) , etc. In the last few years, many novel borate
3+
3
5
4
4
crystals and potential applications have been reported in the
5+
5
3
14
6
7
UV/DeepꢀUV region, such as NaBeB O
K B O Cl ,
23
3
6 ,
3
6
10
groups connected to PO tetrahedra . In the respect of crystal
4
8
9
Cs B SiO , Ba B O F with large powder SHG responses
2
4
9
4
11 20
structure, Kniep and coꢀworkers have systematically studied
about 1.6, 4, 4.6, 10 times of KH PO (KDP) and short UV
24, 25
2
4
borophosphate compounds in their review articles
. Among
cutoff edges of 200, 180, 190, 175 nm, respectively. Recently,
various structures, BꢀOꢀB or BꢀOꢀP connections are observed
1
0
Li Sr(BO )
which is expected as a rival of KBe BO F
2 3 2
4
3 2
1, 12
commonly and made up of BO , BO4 and PO4 tetrahedra
3
1
(
KBBF)
, was obtained. And some phosphates such as
sharing common corners. The PꢀOꢀP connections have also
been found in the borophosphates anions, such as,
1
3
14
Ba P O Cl , Rb Ba (PO ) have also drawn much attention
3
3
10
2
3
3 3
ascribed to their short UV cutoff edges (180, 163 nm) but with
weak SHG effect. Therefore, it is in urgent need of crystals
with good deep UV NLO characteristics.
Cs Cr (BP O )(P O ), CsFe(BP O ), Li Cs B P O
and
2
3
4
14
4
13
3
11
2
2
2
4
15
26
Li M BP O (M=K, Rb) .
3
2
4
14
Owing to the various structures of borophosphates, they
exhibit different properties. Because of the absence of dangling
It is interesting to note that thanks to their complex anionic
groups built of BO , BO , and PO tetrahedra, borophosphates
4
3
4
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bonds of oxygen in BO groups, several borophosphates with relatively small SHG intensities of 0.439 and 0.570 KDP, were
4
2
0
short UV cutoff edge have been found. There are SrBPO
BaBPO5 , BPO4 , Sr BP O , and their cutoff edges are properties accurately, the SrBPO and BaBPO crystals were
,
studied as a demonstration. In order to characterize their
5
2
1
27
28
6
5
20
5
5
below 185, 180, 134 and 200 nm, respectively. Unfortunately, grown by the topꢀseeded solution method. And the additional
the SHG intensities are very weak except BPO . It is also a measurements such as DTA, optical uniformity measurement,
4
common character in borophosphates or phosphates. Luckily, were made. Detailed investigations on the crystal structure,
there are several materials with relatively large SHG responses, electronic structure and optical properties of SrBPO5 and
such as BPO and NH H PO (ADP) with the magnitude of BaBPO have been performed based on DFT calculation. To
4
4
2
4
5
almost double as that of KDP. In fact, for the SHG effect, the insight into the relationship between electronic structure and
magnitude as large as that of KDP is enough for applications in properties, SHGꢀdensity, bandꢀresolved method, realꢀspace
2
9
UV or deepꢀUV wavelength . Therefore, a fully understanding atomꢀcutting method are adopted. The results of calculation
of the mechanism of NLO effects in borophosphate crystals can agree well with those obtained from the experiments but the
help us to design and search new excellent NLO crystals more NLOꢀactive units are not what we expected. Previously, if
efficiently. Generally, compared with the BO group, the BO4 there are BO and PO groups in a compound, the PO groups
3
4
4
4
group in borates is considered to have a neglectable are considered to be the main contributor of SHG because
contribution to SHG effect. While one question remains open borates containing BO groups could possess a SHG effect
4
2
0, 30
that what the role of BO4 is, especially in a series of smaller than that containing BO groups
. Anomalously,
3
borophosphates which only contain BO4 and PO4 anionic through precise analysis based on SHGꢀdensity and bandꢀ
groups. To deeply understand their inner mechanism of NLO resolved methods, we found that the BO groups and metal
4
effects of borophosphates and then guide the design of new cations give the main contribution to SHG effect of MBPO but
5
NLO borophosphate materials, four aspects need to be clarified: not the PO groups as expected. What is more, the PO groups
4
4
i) what the NLOꢀactive units are, ii) how the arrangements of even show the counteractive contribution in BaBPO because of
5
the NLOꢀactive units affect macroscopic NLO effects, iii) what indirect influence from the cations.
the relationship between NLO properties and crystal structures
is, iv) how to enhance the SHG effect.
Until now, several theories have been proposed to help Experimental
chemists and material scientists to design and search new NLO
3
0
Synthesis and Crystal Growth
materials with large SHG effect. The “anionic group theory” ,
proposed by Chen et al., has proven to be successful in Polycrystalline samples of SrBPO5 and BaBPO5 were
searching new NLO borate crystals with large SHG effects, synthesized by traditional solidꢀstate reaction techniques.
such as BBO (5.8
⨉KDP), LBO (about 3⨉KDP), etc. Besides, Analytically pure SrCO (for SrBPO ) or BaCO (for BaBPO ),
3 5 3 5
some materials with large SHG effect which owes to the H BO and NH H PO were weighted as stoichiometric ratio
3
3
4
2
4
combination of secondꢀorder JahnꢀTeller units with diverse and mixed in an agate mortar, and then packed into a platinum
boronꢀoxygen functional units have emerged, typical examples crucible. After preheating at 500 °C for 20 h in muffle furnace,
3
1
32
are Cd BiO(BO ) (6⨉KDP) , Pb B O I (13.5⨉KDP) . In the mixture was cooled to ambient temperature and grinded,
4
3 3
2 5 9
order to get DeepꢀUV NLO crystals, another rigorous pressed. Then, the products were heat up to 900 °C for 72 h (for
prerequisite for UV/DeepꢀUV region is the wide transparence, SrBPO ) and 48 h (for BaBPO ), and cooled to room
5
5
which is expected to unhinder the role of NLOꢀactive structural temperature again. The results were checked by powder Xꢀray
units. Consequently, the introduction of SOJT units usually diffraction, and singleꢀphase powders of SrBPO and BaBPO
5
5
conflicts with the short UV cutoff edge. Then, a modified bond were obtained when repeated heat treatment caused no further
3
3ꢀ35
valence sum method
cutoff edge. Via controlling ambient coordination environment
surrounding the oxygen atoms, the band gap can be increased SrBPO and BaBPO because they are incongruently melting
has been put forward to shorten the changes in the powder Xꢀray diffraction pattern.
The flux method is necessary for the crystal growth of
5
5
2
0, 21
due to the amplitude of eliminating the nonbonding orbitals. compounds. As they were described in previous paper
Notable compounds are Alꢀcontaining borates: K Al B O
topꢀseeded solution growth method was used to grow the single
KAB) , Na Al B O (NABO) and Cs Al B O (CABO), or crystals, BPO (for SrBPO ) and Li P O (BaBPO ) was used
, the
2
2
2
7
3
6
37
(
2
2
2
7
2
2
2
7
4
5
4
2
7
5
3
8
Beꢀcontaining borates: Sr Be B O
(SBBO) , KBBF, as the fluxes. The crystals were then obtained after drawn out of
2
2
2
7
3
9
MM'Be B O F
(M=Na, M’=Ca; M=K, M’=Ca, Sr) . the solution surface, cooled to room temperature at a gradient
2
2
6
Therefore, it is necessary to investigate the insight of of 20 °C/h, then slowly taken out from the furnace.
relationship between crystal structures and NLO properties.
Powder Xꢀray Diffraction
Especially, to our best knowledge, the contribution of anionic
tetrahedral groups in borophosphates for SHG still lacks A Bruker D2 ADVANCE Xꢀray diffract meter equipped with a
investigation. Accordingly, it will be beneficial to design and monochromatic Cu Kα radiation (λ = 1.5418 Å) was used to
search new excellent borophosphate crystals.
Herein, the isostructural compounds SrBPO and BaBPO , 10° to 70° (2θ) was recorded, with a scan step width of 0.02°
measure the powder XꢀRay Diffraction. The angular range from
5
5
which have short UV cutoff edges of 180 and 185 nm and
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4TC02443E
−
1
3
3
and a rate of 10 s step . Theoretical simulation was conducted
on the basis of the single crystal crystallographic data. The
powder Xꢀray diffraction patterns of SrBPO and BaBPO are
(2)
e
d k
α
β
γ
1
2
χαβγ (VE) =
P(αβγ )Im
[
P P P
]
(
+
+
)
)
3
∑∫
3
cv cc' c'v
3
2
4
2hm vcc'
4
π
ω ω
ω ω
vc c'v
cv vc'
.
.
(2)
(3)
5
5
3
3
shown in Figure S1.
(
2)
e
d k
α
β
γ
1
2
χαβγ (VH ) =
P(αβγ )Im
[
P P P
]
(
3
∑∫
3
vv' cv' cv
3
2
4
2
hm vv'c
4
π
ω ω
ω ω
cv v'c
vc cv'
Numerical Calculation Details and Methods
Electronic Structure and Linear Optical Properties
.
(4)
The firstꢀprinciples calculations were performed by the
4
0
CASTEP , a planeꢀwave pseudopotential package based on the
DFT. During the calculation, geometry optimization was
performed using the BFGS minimization technique. The
geometry optimization was converged criterion that the residual
forces on the atoms were less than 0.01 eV/Å, the
Here, α, β, γ are Cartesian components, and v/v', c/c'
denote valence bands (VBs) and conduction bands (CBs). And
α
P(αβγ),
ℏ
ωij and
P
refer to full permutation, the band energy
ij
difference and momentum matrix elements, respectively.
The bandꢀresolved method, revealing the effective values of
individual electronic states in SHG coefficients, comes forward
−
4
displacements of atoms were less than 5×10 Å, and the
−
6
energy change was less than 5.0×10 eV/atom. After a series
of successful tests, the exchangeꢀcorrelation functional we
chose was the generalized gradient approximation (GGA) with
PerdewꢀBurkeꢀErnzerhof (PBE) functional. The normꢀ
conserving pseudopotential (NCP) was used, the valence
(2)
to emphasize the two out of three band indices for χ
(VE) or
αβγ
(2)
χαβγ (VH) in equations (2) and (3). As a result, the SHG effect
of orbital contribution is divided into occupied and unoccupied
bands. Therefore, the analysis scheme exhibits the orbital
(2)
contributions of total χ with level by level vividly. In this
2
6
2
2
1
2
4
2
3
electrons were Srꢀ4s 4p 5s , Bꢀ2s 2p , Oꢀ2s 2p , and Pꢀ2s 2p
respectively. The energy cutoff of plane wave’s basis set was
30 eV, and the Brillouin zone was composed of 4 × 4 × 4
(2)
work, the integral contribution of the bandꢀresolved
χ
in VE
and VH processes was calculated further, which can show the
integral SHG contribution of the corresponding energy region
8
ꢀ
1
MonkhorstꢀPack kꢀpoint sampling with a separation of 0.04 Å .
We kept the default values of the CASTEP code on the aspect
of the other calculation parameters and convergent criteria. The
49
and the contribution of valence bands and conduction bands .
So we can use it to identify and further research the main states
having contribution in a SHG process.
properties of BaBPO were calculated in the same condition
50
5
There is another more vivid method called SHGꢀdensity
2
6
2
except for the valence electrons of Baꢀ5s 5p 6s .
Scissors” correction approximation was implemented by
which can illuminate the SHG response further in complex
electronic structure we researched. The SHGꢀdensity method
was performed by using the effective SHG of each occupied or
unoccupied band as weighting coefficient (after normalized
“
moving all of the conduction bands off valence band with the
gap correction ∆ (the difference between calculated bang gap
4
1ꢀ43
and experimental one). In the scissors operator
, the
(2)
with total VE or VH χ value) to sum the probability densities
momentum matrix elements should be modified. The
correlative renormalization of the moment matrix elements
under the gap correction is satisfied:
of all occupied or unoccupied states. Therefore, this method
ensures that quantum states which cause SHG can be shown
together as these occupied or unoccupied “SHGꢀdensities”,
while states irrelevant to SHG will not be shown. Therefore, the
origin of SHG optical nonlinearity can be highlighted in the real
space through the resulting distribution of such densities. The
methods have been employed successfully to analyze the SHG
4
4
ꢀꢁꢂ → Pꢁꢂꢃ(ωnm + ∆/ℏ(δnc − δmc))/ωnm
Here, (δnc δmc) is the gap correction of the couples of bands
-
4
9
between one valence and one conduction band state. In this
process, as the GGA wave functions approach the true
origin of NLO borates Li Mnꢀ1B2nꢀ1O4nꢀ2(M=Cs/Rb, n=3,4,6)
n
5
1
and Bi ZnOB O .
6
2
2
4
5, 46
quasiparticle wave function
, there is a hypothesis that the
rmn mnmatrix elements is invariant.
Results and Discussion
Methology for Probing SHG Contribution from Electronic
States
Deep Ultraviolet Transmission Spectrum
Ultraviolet cutꢀoff edge of a NLO crystal is an important
indicator of its practical application range. Two large SrBPO5
The soꢀcalled lengthꢀgauge formalism derived by Aversa and
4
7
Sipe was adopted. At a zero frequency, the static secondꢀorder
nonlinear susceptibilities can be ascribed to VirtualꢀHole (VH),
and BaBPO crystals were grown and shown in Figure 1. In
5
order to obtain their cutꢀoff edges accurately, the UV
transmission spectra of SrBPO5 and BaBPO5 crystals were
measured in nitrogen. From the spectra, we can see that the
transparencies of the SrBPO and BaBPO crystals are above
4
8
,
VirtualꢀElectron (VE) and TwoꢀBand (TB) processes
.
(1)
5
5
(
2)
(2)
(2)
Where χαβγ (VE), χαβγ (VH), and χαβγ (two bands) are
computed with the formulas as follows:
8
0% at until 200 and 209 nm, respectively, and decreases
starting from 200 and 209 nm, and reaches zero at 180 and 185
nm, respectively.
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Journal Name
(
a)
(
a)
(
b)
(
b)
Figure 1. The ultraviolet (UV) transmission spectrum and crystal photographs of
SrBO (a) and BaBPO (b).
5
5
Figure 2. The calculated and experimental dispersion curves of birefringence and
experimental refractive index (insert) of SrBPO (a) and BaBPO (b).
5
5
Birefringence and Phase Matching Curves
Type I phase matching (PM) SHG curves of SrBPO and
5
2
0,
BaBPO were calculated according to the Sellmeier equations
According to the pointꢀgroup symmetry of D , the SrBPO and
5
3
5
2
1
as a function of fundamental wavelength (in Figure S3). The
Sellmeier equations indicate that the shortest type I PM SHG
wavelengths are 395 nm for SrBPO and 364 nm for BaBPO
BaBPO5 crystals are uniaxial and there are only two
independent nonzero diagonalizable of dielectric tensors. The
uniaxial properties of them were measured by conoscope using
a crystal slicing perpendicular to optical axis. Interference
patterns were shown in Figure S2. The square of the refractive
5
5
respectively. Obviously, the shorted type I phase matching
PM) SHG wavelength of BaBPO5 is lower than that of
(
SrBPO5.
index
n is equal to the real part of ε (ω). The birefringence can
be obtained from the linear response functions. Figure 2 shows
the calculated and experimental dispersion curves of refractive
indices and birefringence. We can see that the birefringence ∆n,
Thermal Properties
The thermal expansion of a NLO crystal is an important factor
5
2
in its growth and applications . Light absorption by a crystal
causes a thermal gradient that may disturb the laser oscillation,
and high thermal gradients can also lead to crystal fracture. The
defined as no−n , is about 0.018 for SrBPO and 0.026 for
e 5
BaBPO at 1064 nm. The calculated results are consistent well
5
with the experimental values. As shown in Figure 2, because of
small dispersion of refractive indices, we can predict that
SrBPO and BaBPO could better fulfil the phaseꢀmatching
thermal expansions of aꢀ and cꢀaxis of SrBPO and BaBPO are
5
5
measured ranging 50 to 300 °C, shown in Figure 3. From this
figure, the thermal expansion curves demonstrate that no
anomaly within the measuring temperature range is observed
and the curves are linear within this range. The average linear
5
5
conditions.
4
| J. Name., 2012, 00, 1-3
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thermal expansion coefficients of the aꢀ and cꢀoriented in wavelength of 632.8 nm were measured in this experiment. The
ꢀ
6
ꢀ6
ꢀ5
SrBPO and BaBPO crystals are 7.66
⨉
10 /°C, 7.81
⨉
10 /°C values of optical uniformity are 1.26
×
10 /cm (SrBPO ) and
5
5
5
ꢀ
6
ꢀ6
ꢀ5
and 8.72
⨉
10 /°C, 7.82
⨉
10 /°C, respectively.
2.48
×
10 /cm (BaBPO ), which indicates that both crystals
5
From the crystal structure of both compounds, one can have high qualities.
expect that the thermal expansion coefficients of both
compounds would show a small anisotropy along aꢀ and cꢀaxes.
This is in agreement with our experimental results, thus both
Electronic Structure
crystals are not cracked during the crystal growth.
The band structures of SrBPO5 and BaBPO5 are also very
The DTA curves of SrBPO and BaBPO were shown in similar. SrBPO and BaBPO are direct gap (Figure S6) crystals
5
5
5
5
Figure S4 measured from 900 to 1300 °C for SrBPO and 900 with the calculated band gaps of 6.15 and 5.89 eV, as shown in
5
to 1250 °C for BaBPO . As shown in the DTA curves, there are Table 1. Comparing with the optical experimental gaps 6.44
5
two endothermic peak at 1220 and 1238 °C for SrBPO and two (SrBPO ) and 6.14 eV (BaBPO ), the calculated gaps were in
5
5
5
endothermic peaks at 1203 and 1121 °C for BaBPO . These agreement very well. And the very small difference between the
5
results further indicate that both compounds are incongruent calculated and the experimental ones is because of the
melting compounds. Therefore, the flux method is necessary for discontinuity of exchangeꢀcorrelation energy. To get linear and
the purpose of their crystal growth.
NLO optical properties more accurately, we need the
resonances occurring at the correct energies. The scissors
operator will be applied to the computation of optical
properties.
(
a)
Figure 4 shows the partial density of states (PDOS) of
SrBPO and BaBPO . For SrBPO the main occupant is Oꢀ2p
5
5
5,
orbital in the VBs near the Fermi surface and the Srꢀ4p orbitals
at the bottom of the CBs, which suggests that the SrꢀO bonds
determine the band gap. Differently, in BaBPO , Baꢀ5d orbital
5
is dominant at the bottom of CBs. It means that the BaꢀO bond
controls the band gap of BaBPO . Therefore the difference
5
between electronic structures of Sr and Ba makes a red shift
about 0.17 eV for BaBPO comparing with SrBPO .
5
5
5 5
Table 1. The calculated and experimental band gaps of SrBPO and BaBPO .
Crystal
SrBPO (eV)
BaBPO (eV)
Calcd.
6.15
Expel.
6.44
Difference
0.29
(
b)
5
5
5.98
6.14
0.16
(
a)
5 5
Figure 3.The thermal expansions of a- and c-axis of SrBPO (a) and BaBPO (b).
Optical Uniformity Measurement
Optical homogeneity of the crystal (Figure S5) is also an
important indicator of practical application. Therefore, a single
3
crystal with a size of 3.50 × 3.50 × 3.45 mm of SrBPO and a
5
3
size of 3.50 × 3.50 × 3.45 mm of BaBPO with a laser of
5
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Figure 4 shows the partial density of states (PDOS) and
(
2)
integral of bandꢀresolved
χ
of SrBPO and BaBPO , which
5 5
(
b)
counts the integral contribution of SHG effect in the energy
representation. Here we choose BaBPO as a demonstration to
5
locate certain orbital of atomic clusters using the bandꢀresolved
method. According to the variation of integral of bandꢀresolved
(
2)
χ
(green curves in Figure 4), the PDOS was divided into
several regions in energy. In the region I (ꢀ3.81 eV~ ꢀ2.01 eV)
in Figure 4a, the electrons of O1ꢀ z (the nonbonding orbital)
and the bonding orbital of O2 occupy this energy region and the
p
p
(
2)
integral of bandꢀresolved
contribution of this region is negative. Meanwhile, the electrons
of the nonbonding orbital of O2 and O3 atom occupy the
region II (ꢀ2.01~0.00 eV), and the integral of bandꢀresolved
χ
decrease, which suggests that the
p
(
2)
χ
increases which corresponds to positive contributions to SHG
effect. Similarly, in the region III (6.3~8.6 eV), the most part is
Baꢀ5d orbital and it is the main positive contribution in CBs
(
2)
according to the curve of integral of bandꢀresolved
χ .
Figure 4. The PDOS of SrBPO
5 5
(a) and BaBPO (b). I-V regions refer to the
Except for the contribution of electronic states within
negative contribution region, the main contribution region in VBs, the main
contribution region in CBs, the offset contribution to the SHG coefficient, the energy band framework, the electronic structure subsystem
second contribution in CBs.
which causes SHG can be visualized via the SHGꢀdensity
method. According to the results shown in Table 2, the VE
NLO Properties
process would be analysed due to its domination in the SHG
Interestingly, different microscopic
response in BaBPO5.
We calculated SHG coefficients dij under the static limit within
the length gauge. According to the D3 pointꢀgroup symmetry
and Kleinman symmetry, only d11 should be determined. The
experimental SHG intensities and computed ones at
combinations of exchangeꢀcorrelation functional and
pseudopotentials are shown in Table 2. The result, calculated
on the condition of GGA and NCP, agrees well with the
experimental result, which proves the validity of our studies on
SrBPO5 and BaBPO5 with the GGA planeꢀwave
pseudopotential method.
groups or atoms exhibit different contributions: the PO groups
4
give a negative contribution (Figure 5a) to SHG effect, while
the BO groups and metal cations own positive contributions
4
(
Figures 6bꢀ6d). To clarify such counteraction from different
parts, we studied the main contribution region and
corresponding orbitals in energy. Figure 5a shows the
counteractive contributor in the region I which corresponds to
2
p orbitals of oxygen in PO . We can see that orbital and
4
negative SHG density have a great overlap. In other words, the
pꢀelectron orbital of oxygen in PO groups, which play the
4
Table 2. Comparisons of the calculated SHG coefficients with a correction of
the band gap and experimental SHG effect of SrBPO
counteractive role of SHG effect. Oppositely, we can see the
5
and BaBPO
5
.
positive contributions which come from the BO groups and are
4
corresponding to
p orbitals of O3 in Figure 5b. Figure 5c
Crystal
Expel.
Calcd. d11(pm/V)
0.218(0.56KDP)
0.157(0.40KDP)
VE
VH
ꢀ0.05
0.02
shows the contribution of the region III (6.3 eV~8.6 eV). The
SHG density sprinkles on Baꢀ5d orbital but not on the Bꢀ2p and
Pꢀ3p orbital, which indicates that the Baꢀ5d orbital is the main
SrBPO
5
0.57KDP
0.43KDP
0.268
0.137
BaBPO
5
contributor in this region. Similarly, in the region V, the antiꢀ
orbital of the BꢀO bond is the main contributor in SHG density,
σ
SHG Response in MBPO5
which illustrates the importance of BO groups as shown in
Figure 5d. In conclusion, the PO4 groups have negative
4
Distribution of the Electron States to SHG Response
For the process of NLO effects, the shortꢀrange forces rather contributions while the BO
than longꢀrange interactions play a decisive role. In other are the main contributors to SHG effect of BaBPO
words, in the NLO effects, the electron motion may be regarded
The same methology was employed to analyze the SHG
as confined to small regions and any NLO susceptibility (or response of SrBPO . The main difference from BaBPO is that
secondꢀorder susceptibility) in crystals is a localized effect the counteraction contributions almost vanish in the region I.
arising from the action of incident photons on the electrons in Meanwhile, we can find the same SHG response of BO like
in region III, IV and V and this response
distribution of the electron states in SHG response, the integral has not been observed in PO . That is to say, the BO group is
contribution of the bandꢀresolved χ in VE and VH processes still the main contributor of SHG effect. In addition, compared
and the SHGꢀdensity were calculated.
the region III of SrBPO and BaBPO in Figure 4, owing to the
absence of orbital of the Sr atom (Figure 4b), the DOS of the
groups and the Ba metal cations
4
.
5
5
5
4
3
0
certain orbitals of atomic clusters . In this work, to study the Figure 5c in SrBPO
5
4
4
4
9
(2)
5
5
d
B atom and the P atom increases. This change is bound to
6
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ARTICLE
DOI: 10.1039/C4TC02443E
whittle the contribution of metal cations and strengthen the section. Therefore, the BO groups become the main role in
4
contribution of BO . This phenomenon will be further SHG effect of SrBPO5.
4
confirmed by the realꢀspace atomꢀcutting method in the next
(
a)
(b)
(
c)
(d)
Figure 5. The SHE-density of VE process for BaBPO
5
. (a) The negative contribution in SHG density of occupied states (orange) and the corresponding orbitals (green),
b) The positive contribution in SHG density of occupied states (orange) and the corresponding orbitals (green), (c) The positive contribution of unoccupied states
red) of Ba and the corresponding orbitals (green), (d) the positive contribution of unoccupied states (red) of BO and the corresponding orbitals (green).
(
(
4
)
55
The Significance of
d
Orbital
AMoO (IO )(A=K, Rb, Cs , AGa(SeO ) (A = Li, Na, K, and
3 3 3 2
5
6
57
Although we found the main contribution of SHG effect in Cs) , A Ti(IO ) (A=Li, Na, K, Rb, Cs, Tl) and
MBPO , there is still one question: why the counteractive MM’Be B O F (M=Na, M’=Ca; M=K, M’=Ca, Sr) . In this
contribution vanishes in SrBPO . In the matter of crystal work, we employed Gaussian to calculate the dipole moments
2
3 6
5
8
5
2
2
6
5
9
5
structure, SrBPO and BaBPO are isostructural and possess of MO polyhedra and the result suggests that there is no
5
5
similar lattice parameters: a=7.12 Å, c=6.96 Å and a=6.84 Å, difference in SrBPO and BaBPO . Therefore, we think that the
5
5
c=6.79 Å, respectively. The bond lengths and angles of BO4 radius difference of cation will not cause the local distortion of
and PO are almost the same. The radii of Ba and Sr atom are the tetrahedral and overall framework structures and also
4
2
.78 Å and 2.45 Å. The radius difference of cation may cause cannot cause different SHG effect of metal and PO group. In
4
the local distortion of the polyhedral and overall framework fact, the dangling bond of the terminal oxygen exists only in
structures in some compounds such as ACuTe O (A = PO and is easily influenced by the metal cations, which means
2
7
4
2
+ 53
54
Sr2+,Ba2+,or
Pb ) ,
ANaNbOF5
(A=K,
Cs) , that metal cations make great difference to PO rather than BO .
4 4
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Journal Name
5 5
Figure 6. The LUMO of SrBPO (a) and BaBPO (b).
The influence of the metal cations to the terminal oxygen (O2
for BaBPO and O1 for SrBPO ) and boronꢀphosphorus bridge
5
5
oxygen (O1 for BaBPO and O2 for SrBPO ) in PO can be Spatial Contributions of Groups and Ions to SHG Response
5
5
4
reflected in the lowest unoccupied molecular orbital (LUMO,
Figure 6). It is noted that the interaction between terminal
Strangely enough, it is believed previously that if there are
PO4 and BO4 together, the PO4 group should be the main
contributor to the SHG effect . Perhaps they do not take
oxygen or boronꢀphosphorus bridge oxygen in PO and metal
20
4
cation is stronger than that between boronꢀboron bridge oxygen
microstructure into consideration. In order to verify the
O3 in BO and metal cation. More importantly, the electronic
4
contribution of Ba and BO groups and the counteractive of
4
structure and PDOS show that
d
orbitals of the Ba cations
51
PO , the realꢀspace atomꢀcutting method was employed in our
4
dominate in LUMO, while B and P dominate in LUMO in
work. In the “cutting”, it is of importance to determine the
suitable cutting radii of Ba, B, P and O1ꢀO3. Owing to the
chargeꢀdensity distributions and the overlap behaviours in these
compounds, the cutting radii of Sr, Ba, B, P, O are 1.80 , 1.80,
SrBPO . This suggests that, the interaction between oxygens
5
and Ba cations in BaBPO is stronger than that in SrBPO .
5
5
Thus, the Baꢀ5d orbital is closely related to the properties of
PO . And owing to the absence of orbital of Sr, the
counteractive contribution from PO vanishes in SrBPO . There
d
4
0
.82, 1.52, 1.11 Å, respectively. According to Rietveld
4
5
technique, for BaBPO , the anionic groups are BO and PO ,
5
4
4
is no wonder that the contribution of PO in SHG effect in
60
4
not BPO7 and its decomposed products are Ba BP O and
3
3
12
SrBPO and BaBPO are different. Meanwhile, the Ba cations
5
5
Ba P O . Hence, BO or PO can be regarded as a group to be
2
2
7
4
4
because of their
d orbitals have more contribution in SHG
cut and the bridge oxygen is assigned to PO group because it
4
effect in BaBPO5.
has stronger interaction with phosphorus than boron according
to the decomposed products. Consequently, the contributions
are: d11(Ba) = ꢀ0.11 pm/V, d11 (PO ) = 0.20 pm/V, d11(BO ) = ꢀ
4
4
(
a)
0
.26 pm/V. It is worth noting that the total value of d11 of
BaBPO is negative (For convenience, we convert the value of
5
d11 to positive in the bandꢀresolved analysis of Figure 4 and
Table 2). So the contribution of PO4 is counteractive. The
cutting results of SrBPO are as expected: d11(PO ) = 0.03
5
4
pm/V,
d11(BO ) = 0.20 pm/V, d11(Sr) = 0.04 pm/V.
4
Consequently, in SrBPO , the PO groups have a very tiny
5
4
contribution.
The results obtained from the realꢀspace atomꢀcutting
method confirm the contributions of different groups or ions.
The curious effect from the metal cations is revealed further: i)
direct contribution from the Ba ions is more apparent than that
from the Sr ions, which is due to the difference from the
electron states; ii) indirect influence from metal cations to the
BO and PO groups leads to dominant action of BO in both
4
4
4
crystals, and a counteractive contribution of the PO groups in
4
BaBPO but a positively tiny contribution in SrBPO .
5 5
(
b)
Conclusions
In summary, electronic structure and optical properties of
MBPO5 were studied theoretically and experimentally. The
large crystal of SrBPO and BaBPO have been successfully
5
5
grown by topꢀseeded solution growth method. Additional
measurements, such as optical uniformity and thermal
expansion measurements, were made. The results demonstrate
that SrBPO and BaBPO5 have good optical qualities. We
5
employed the bandꢀresolved method, SHGꢀdensity method,
realꢀspace atomꢀcutting method and bandꢀresolved method to
analyze the NLO properties. The result suggests that the main
contribution of SHG in BaBPO is BO and the Ba cations
5
4
while the main NLOꢀactive units in SrBPO are almost BO
5
4
groups. Fallaciously, the PO groups do not undertake the main
4
role. On the contrary, it has the counteractive effect in BaBPO5.
Remarkably, through analysing the mechanism of the origin of
8
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ARTICLE
DOI: 10.1039/C4TC02443E
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4
4
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0 | J. Name., 2012, 00, 1-3
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Page 11 of 11
Journal of Materials Chemistry C
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DOI: 10.1039/C4TC02443E
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Anomalous Second Harmonic Generation (SHG) Response
in MBPO (M=Sr, Ba)
5
a,b
a,b
,a
a,b
Bing-Hua Lei, Qun Jing, Zhihua Yang,* Bingbing Zhang, Shilie Pan*
,a
a
Key Laboratory of Functional Materials and Devices for Special Environments of CAS; Xinjiang
Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of
Physics and Chemistry of CAS, Urumqi 830011, China. Fax: (86)-991-3838957; Tel:
(
86)-991-3810816;
b
University of Chinese Academy of Sciences, Beijing 100049, China
*
E-mail: zhyang@ms.xjb.ac.cn, slpan@ms.xjb.ac.cn
Tel: (86)-991-3810816
Fax: (86)-991-3838957
4
Based on SHG-density and band-resolve analysis, we find that BO groups and metal
cation are the main contribution in SHG effect of MBPO
groups as previously expected.
5 4
(M=Ba, Sr) but not the PO