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Bulk Modulus01:21

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The bulk modulus is a scientific term used to describe a material's resistance to uniform compression. It is the proportionality constant that links a change in pressure to the resulting relative volume change.
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A single nucleotide polymorphism or SNP is a single nucleotide variation at a specific genomic position in a large population. It is the most prevalent type of sequence variation found in the human genome. Point mutations that occur in more than 1% of the population qualify as SNPs. These are present once every 1000 nucleotides on an average in the human genome. Replacement of a purine with another purine (A/G) or a pyrimidine with another pyrimidine (C/T) is known as a transition. In contrast,...
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Polymorphism refers to the existence of a drug substance in multiple crystalline forms, known as polymorphs. Recently, this term has been expanded to include solvates (forms containing a solvent), amorphous forms (non-crystalline forms), and desolvated solvates (forms from which the solvent has been removed).
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The moisture content of aggregates is a crucial factor in construction, particularly in concrete mixing, as it influences the total water required in the mix. Moisture content represents the water coated on the exterior surface of the aggregate existing in a saturated and surface-dry condition. The total water content of a moist aggregate is the sum of its moisture content and water absorption.
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Changes in polymorphic forms can significantly influence the bioavailability of poorly soluble drugs. Although the FDA defines pharmaceutical equivalence based on having the same active ingredient, dosage form, and route of administration, it does not automatically disqualify products with different polymorphic forms. This means two products with different polymorphs can still be deemed pharmaceutically equivalent. However, polymorphic differences can affect properties like wettability,...
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Polymorphism of bulk boron nitride.

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The ground-state phase of boron nitride (BN) is cubic, not hexagonal. Hexagonal boron nitride becomes entropically stabilized above room temperature, with a new monoclinic phase also identified.

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Area of Science:

  • Materials Science
  • Condensed Matter Physics
  • Computational Chemistry

Background:

  • Boron nitride (BN) exhibits exceptional properties, but the thermodynamic stability of its cubic and hexagonal polymorphs remains unresolved.
  • Kinetic factors in high-pressure/temperature synthesis and complex interatomic forces complicate theoretical predictions.
  • This knowledge gap impedes technological applications and fundamental scientific understanding.

Purpose of the Study:

  • To accurately determine the relative thermodynamic stability of boron nitride polymorphs using advanced theoretical methods.
  • To resolve discrepancies between theoretical predictions and experimental observations of BN phases.
  • To provide a foundation for future research and development of BN materials.

Main Methods:

  • Employed high-level first-principles theories, specifically the adiabatic-connection fluctuation-dissipation theorem in the random phase approximation.
  • Calculated energy differences between BN polymorphs with unprecedented accuracy, overcoming limitations of previous theoretical studies.
  • Investigated the role of temperature and entropic stabilization on phase transitions.

Main Results:

  • Established that the cubic phase is the ground-state of boron nitride.
  • Demonstrated that the hexagonal boron nitride phase is entropically stabilized at temperatures slightly above ambient conditions (335 ± 30 K).
  • Identified a competitive low-symmetry monoclinic phase, potentially explaining the "compressed h-BN" experimental observation.

Conclusions:

  • The cubic phase is the most stable form of boron nitride under ground-state conditions.
  • Hexagonal boron nitride's prevalence is due to entropic stabilization at accessible temperatures.
  • The findings necessitate new experimental investigations and promote the use of advanced theories for van der Waals materials.